Learn How Transmissions Work Overview

    Manual transmissions often feature a driver-operated clutch and a movable gear stick. Most
    automobile manual transmissions allow the driver to select any forward gear ratio ("gear")
    at any time, but some, such as those commonly mounted on motorcycles and some types
    of racing cars, only allow the driver to select the next-higher or next-lower gear. This type
    of transmission is sometimes called a sequential manual transmission. Sequential
    transmissions are commonly used in auto racing for their ability to make quick shifts.

    Manual transmissions are characterized by gear ratios that are selectable by locking selected
    gear pairs to the output shaft inside the transmission. Conversely, most automatic transmissions
    feature epicyclic (planetary) gearing controlled by brake bands and/or clutch packs to select
    gear ratio. Automatic transmissions that allow the driver to manually select the current gear
    are called Manumatics. A manual-style transmission operated by computer is often called an
    automated transmission rather than an automatic.

    Contemporary automobile manual transmissions typically use four to six forward gears and one
    reverse gear, although automobile manual transmissions have been built with as few as two and
    as many as eight gears. Transmission for heavy trucks and other heavy equipment usually have
    at least 9 gears so the transmission can offer both a wide range of gears and close gear ratios to
    keep the engine running in the power band. Some heavy vehicle transmissions have dozens of
    gears, but many are duplicates, introduced as an accident of combining gear sets, or introduced
    to simplify shifting. Some manuals are referred to by the number of forward gears they offer
    (e.g., 5-speed) as a way of distinguishing between automatic or other available manual
    transmissions. Similarly, a 5-speed automatic transmission is referred to as a "5-speed
    automatic."

    Unsynchronized transmission

    The earliest form of a manual transmission is thought to have been invented by Louis-René
    Panhard and Emile Levassor in the late 19th century. This type of transmission offered multiple
    gear ratios and, in most cases, reverse. The gears were typically engaged by sliding them on
    their shafts (hence the phrase shifting gears), which required a lot of careful timing and throttle
    manipulation when shifting, so that the gears would be spinning at roughly the same speed
    when engaged; otherwise, the teeth would refuse to mesh. These transmissions are called
    sliding mesh transmissions and sometimes called a crash box, because of the difficulty in
    changing gears and the loud grinding sound that often accompanied. Newer manual
    transmissions on cars, instead have all gears mesh at all times; these are referred to as
    constant-mesh transmissions, with "synchro-mesh" being a further refinement of the
    constant mesh principle.

    In both types, a particular gear combination can only be engaged when the two parts to engage
    (either gears or clutches) are at the same speed. To shift to a higher gear, the transmission is
    put in neutral and the engine allowed to slow down until the transmission parts for the next gear
    are at a proper speed to engage. The vehicle also slows while in neutral and that slows other
    transmission parts, so the time in neutral depends on the grade, wind, and other such factors.
    To shift to a lower gear, the transmission is put in neutral and the throttle is used to speed up
    the engine and thus the relevant transmission parts, to match speeds for engaging the next
    lower gear. For both upshifts and downshifts, the clutch is released (engaged) while in neutral.
    Some drivers use the clutch only for starting from a stop, and shifts are done without the clutch.
    Other drivers will depress (disengage) the clutch, shift to neutral, then engage the clutch
    momentarily to force transmission parts to match the engine speed, then depress the clutch
    again to shift to the next gear, a process called double clutching. Double clutching is easier
    to get smooth, as speeds that are close but not quite matched need to speed up or slow
    down only transmission parts, whereas with the clutch engaged to the engine, mismatched
    speeds are fighting the rotational inertia and power of the engine.

    Even though automobile and light truck transmissions are now almost universally synchronised,
    transmissions for heavy trucks and machinery, motorcycles, and for dedicated racing are usually
    not. Non-synchronized transmission designs are used for several reasons. The friction material,
    such as brass, in synchronizers is more prone to wear and breakage than gears, which are forged
    steel, and the simplicity of the mechanism improves reliability and reduces cost. In addition, the
    process of shifting a synchromesh transmission is slower than that of shifting a non-synchromesh
    transmission. For racing of production-based transmissions, sometimes half the teeth (or dogs)
    on the synchros are removed to speed the shifting process, at the expense of greater wear.

    Heavy duty trucks often use unsynchronized transmissions. Military trucks usually have synchronized
    transmissions, allowing untrained personnel to operate them in emergencies. In the United States,
    traffic safety rules refer to non-synchronous transmissions in classes of larger commercial motor
    vehicles. In Europe, heavy duty trucks use synchronized gearboxes as standard.

    Similarly, most modern motorcycles use unsynchronized transmissions as synchronizers are
    generally not necessary or desirable. Their low gear inertias and higher strengths mean that
    forcing the gears to alter speed is not damaging, and the pedal operated selector on modern
    motorcycles is not conducive to having the long shift time of a synchronized gearbox. Because
    of this, it is necessary to synchronize gear speeds by blipping the throttle when shifting into
    a lower gear on a motorcycle.

    Synchronized transmission

    Most modern cars are fitted with a synchronized gear box. Transmission gears are always in mesh
    and rotating, but gears on one shaft can freely rotate or be locked to the shaft. The locking
    mechanism for a gear consists of a collar (or dog collar) on the shaft which is able to slide
    sideways so that teeth (or dogs) on its inner surface bridge two circular rings with teeth on
    their outer circumference: one attached to the gear, one to the shaft. When the rings are
    bridged by the collar, that particular gear is rotationally locked to the shaft and determines the
    output speed of the transmission. The gearshift lever manipulates the collars using a set of
    linkages, so arranged so that one collar may be permitted to lock only one gear at any one time;
    when "shifting gears," the locking collar from one gear is disengaged before that of another
    engaged. One collar often serves for two gears; sliding in one direction selects one transmission
    speed, in the other direction selects another.

    In a synchromesh gearbox, to correctly match the speed of the gear to that of the shaft as the
    gear is engaged, the collar initially applies a force to a cone-shaped brass clutch attached to
    the gear, which brings the speeds to match prior to the collar locking into place. The collar is
    prevented from bridging the locking rings when the speeds are mismatched by synchro rings
    (also called blocker rings or baulk rings, with the latter being spelled balk in the U.S.). The
    synchro ring rotates slightly due to the frictional torque from the cone clutch. In this position,
    the dog clutch is prevented from engaging. The brass clutch ring gradually causes parts to spin
    at the same speed. When they do spin the same speed, there is no more torque from the
    cone clutch, and the dog clutch is allowed to fall in to engagement. In a modern gearbox,
    the action of all of these components is so smooth and fast it is hardly noticed.

    The modern cone system was developed by Porsche and introduced in the 1952 Porsche 356;
    cone synchronisers were called Porsche-type for many years after this. In the early 1950s, only
    the second-third shift was synchromesh in most cars, requiring only a single synchro and a
    simple linkage; drivers' manuals in cars suggested that if the driver needed to shift from second
    to first, it was best to come to a complete stop then shift into first and start up again. With
    continuing sophistication of mechanical development, however, fully synchromesh transmissions
    with three speeds, then four speeds, and then five speeds, became universal by the 1980s.
    Many modern manual transmission cars, especially sports cars, now offer six speeds.

    Reverse gear, however, is usually not synchromesh, as there is only one reverse gear in the
    normal automotive transmission and changing gears into reverse while moving is not required.
    Among the cars that have synchromesh in reverse are the 1995-2000 Ford Contour and
    Mercury Mystique, '00-'05 Chevrolet Cavalier, Mercedes 190 2.3-16, the V6 equipped Alfa
    Romeo GTV/Spider (916),[1] certain Chrysler, Jeep, and GM products which use the New
    Venture NV3500 and NV3550 units, the European Ford Sierra and Granada/Scorpio equipped
    with the MT75 gearbox, the Volvo 850, and almost all Lamborghinis and BMWs.

    Internals

    Shafts

    Like other transmissions, a manual transmission has several shafts with various gears and
    other components attached to them. Typically, a rear-wheel-drive transmission has three
    shafts: an input shaft, a countershaft and an output shaft. The countershaft is sometimes
    called a layshaft.

    In a rear-wheel-drive transmission, the input and output shaft lie along the same line, and
    may in fact be combined into a single shaft within the transmission. This single shaft is
    called a mainshaft. The input and output ends of this combined shaft rotate independently,
    at different speeds, which is possible because one piece slides into a hollow bore in the
    other piece, where it is supported by a bearing. Sometimes the term mainshaft refers to
    just the input shaft or just the output shaft, rather than the entire assembly.

    In some transmissions, it's possible for the input and output components of the mainshaft
    to be locked together to create a 1:1 gear ratio, causing the power flow to bypass the
    countershaft. The mainshaft then behaves like a single, solid shaft, a situation referred
    to as direct drive.

    Even in transmissions that do not feature direct drive, it's an advantage for the input and
    output to lie along the same line, because this reduces the amount of torsion that the
    transmission case has to bear.

    Under one possible design, the transmission's input shaft has just one pinion gear, which
    drives the countershaft. Along the countershaft are mounted gears of various sizes,
    which rotate when the input shaft rotates. These gears correspond to the forward
    speeds and reverse. Each of the forward gears on the countershaft is permanently
    meshed with a corresponding gear on the output shaft. However, these driven gears
    are not rigidly attached to the output shaft: although the shaft runs through them,
    they spin independently of it, which is made possible by bearings in their hubs. Reverse
    is typically implemented differently, see the section on Reverse.

    Most front-wheel-drive transmissions for transverse engine mounting are designed
    differently. For one thing, they have an integral final drive and differential. For another,
    they usually have only two shafts; input and countershaft, sometimes called input and
    output. The input shaft runs the whole length of the gearbox, and there is no separate
    input pinion. At the end of the second (counter/output) shaft is a pinion gear that
    mates with the ring gear on the differential.

    Front-wheel and rear-wheel-drive transmissions operate similarly. When the transmission
    is in neutral, and the clutch is disengaged, the input shaft, clutch disk and countershaft
    can continue to rotate under their own inertia. In this state, the engine, the input shaft
    and clutch, and the output shaft all rotate independently.

    Dog clutch

    Among many different types of clutches, a dog clutch provides non-slip coupling of two rotating members.
    It is not at all suited to intentional slipping, in contrast with the foot-operated friction clutch of a
    manual-transmission car.

    The gear selector does not engage or disengage the actual gear teeth which are permanently meshed.
    Rather, the action of the gear selector is to lock one of the freely spinning gears to the shaft that runs
    through its hub. The shaft then spins together with that gear. The output shaft's speed relative to
    the countershaft is determined by the ratio of the two gears: the one permanently attached to the
    countershaft, and that gear's mate which is now locked to the output shaft.

    Locking the output shaft with a gear is achieved by means of a dog clutch selector. The dog clutch
    is a sliding selector mechanism which is splined to the output shaft, meaning that its hub has teeth
    that fit into slots (splines) on the shaft, forcing that shaft to rotate with it. However, the splines allow
    the selector to move back and forth on the shaft, which happens when it is pushed by a selector fork
    that is linked to the gear lever. The fork does not rotate, so it is attached to a collar bearing on the
    selector. The selector is typically symmetric: it slides between two gears and has a synchromesh and
    teeth on each side in order to lock either gear to the shaft.

    Synchromesh

    If the teeth, the so-called dog teeth, make contact with the gear, but the two parts are spinning at
    different speeds, the teeth will fail to engage and a loud grinding sound will be heard as they clatter
    together. For this reason, a modern dog clutch in an automobile has a synchronizer mechanism or
    synchromesh, which consists of a cone clutch and blocking ring. Before the teeth can engage,
    the cone clutch engages first which brings the selector and gear to the same speed using friction.
    Moreover, until synchronization occurs, the teeth are prevented from making contact, because
    further motion of the selector is prevented by a blocker (or baulk) ring. When synchronization occurs,
    friction on the blocker ring is relieved and it twists slightly, bringing into alignment certain grooves
    and notches that allow further passage of the selector which brings the teeth together.
    Of course, the exact design of the synchronizer varies from manufacturer to manufacturer.

    The synchronizer[2] has to overcome the momentum of the entire input shaft and clutch disk
    when it is changing shaft rpm to match the new gear ratio. It can be abused by exposure to the
    momentum and power of the engine itself, which is what happens when attempts are made to
    select a gear without fully disengaging the clutch. This causes extra wear on the rings and sleeves,
    reducing their service life. When an experimenting driver tries to "match the revs" on a synchronized
    transmission and force it into gear without using the clutch, the synchronizer will make up for any
    discrepancy in RPM. The success in engaging the gear without clutching can deceive the driver into
    thinking that the RPM of the layshaft and transmission were actually exactly matched. Nevertheless,
    approximate rev. matching with clutching can decrease the general change between layshaft and
    transmission and decrease synchro wear.

    Reverse

    The previous discussion normally applies only to the forward gears. The implementation
    of the reverse gear is usually different, implemented in the following way to reduce the
    cost of the transmission. Reverse is also a pair of gears: one gear on the countershaft
    and one on the output shaft. However, whereas all the forward gears are always
    meshed together, there is a gap between the reverse gears. Moreover, they are both
    attached to their shafts: neither one rotates freely about the shaft. What happens when
    reverse is selected is that a small gear, called an idler gear or reverse idler, is slid between
    them. The idler has teeth which mesh with both gears, and thus it couples these gears
    together and reverses the direction of rotation without changing the gear ratio.

    In other words, when reverse gear is selected, it is in fact actual gear teeth that are
    being meshed, with no aid from a synchronization mechanism. For this reason, the
    output shaft must not be rotating when reverse is selected: the car must be stopped.
    In order that reverse can be selected without grinding even if the input shaft is
    spinning inertially, there may be a mechanism to stop the input shaft from spinning.
    The driver brings the vehicle to a stop, and selects reverse. As that selection is made,
    some mechanism in the transmission stops the input shaft. Both gears are stopped
    and the idler can be inserted between them. There is a clear description of such a
    mechanism in the Honda Civic 1996-1998 Service Manual, which refers to it as
    a "noise reduction system":

Whenever the clutch pedal is depressed to shift into reverse, the mainshaft continues to rotate because of its inertia. The resulting speed difference between mainshaft and reverse idler gear produces gear noise [grinding]. The reverse gear noise reduction system employs a cam plate which was added to the reverse shift holder. When shifting into reverse, the 5th/reverse shift piece, connected to the shift lever, rotates the cam plate. This causes the 5th synchro set to stop the rotating mainshaft.

—(13-4)

    A reverse gear implemented this way makes a loud whining sound, which is not normally
    heard in the forward gears. The teeth on the forward gears of most consumer
    automobiles are helically cut. When helical gears rotate, there is constant contact
    between gears, which results in quiet operation. In spite of all forward gears being
    always meshed, they do not make a sound that can be easily heard above the engine
    noise. By contrast, most reverse gears are spur gears, meaning that they have straight
    teeth, in order to allow for the sliding engagement of the idler, which is difficult with
    helical gears. The teeth of spur gears clatter together when the gears spin, generating
    a characteristic whine.

    It is clear that the spur gear design of reverse gear represents some compromises
    (less robust, unsynchronized engagement and loud noise) which are acceptable due
    to the relatively small amount of driving that takes place in reverse. The gearbox of
    the classic SAAB 900 is a notable example of a gearbox with a helical reverse gear
    engaged in the same unsynchronized manner as the spur gears described above. Its
    strange design allows reverse to share cogs with first gear, and is exceptionally quiet,
    but results in difficult engagement and unreliable operation. However, many modern
    transmissions now include a reverse gear synchronizer and helical gearing.

    Design variations

    Gear variety

    Older cars were generally equipped with 3 or 4-speed transmissions for high performance
    models and 5-speeds for the most sophisticated of automobiles; in the 1970s, 5-speed
    transmissions began to appear in low priced mass market automobiles and even compact
    pickup trucks, pioneered by Toyota (who advertised the fact by giving each model the
    suffix SR5 as it acquired the fifth speed). 6-speed transmissions started to emerge in
    high performance vehicles in the early 1990s. In three or four speed transmissions, in
    most cases, the topmost gear is direct (i.e., a 1:1 ratio). For five speed or higher
    transmissions, the highest gear is usually an overdrive gear, with a ratio of less than 1:1.

    Today, mass market automotive manual transmissions are nearly all 6-speeds with
    5-speed transmissions available in smaller vehicles and entry-level models. Recently
    Porsche announced the next-generation 911 will be available with a 7-speed manual
    transmission, the first of its kind for a normal automobile[3][4] with the first six gear
    ratios the same as the 6-speed gearbox and the 7th gear being of a higher ratio.

    Initially the Tesla Roadster was intended to have a purpose built two-speed manual
    transmission[5] but this gearbox proved to be problematic and was later replaced
    with a fixed-ratio transmission.

    External overdrive

    On earlier models with three or four forward speeds, the lack of an overdrive ratio for relaxed and
    fuel-efficient highway cruising was often filled by incorporating a separate overdrive unit in the
    rear housing of the transmission. This unit was separately actuated by a knob or button, often
    incorporated into the gearshift knob.

    Shaft and gear configuration

    On a conventional rear-drive transmission, there are three basic shafts; the input, the output,
    and the countershaft. The input and output together are called the mainshaft, since they are
    joined inside the transmission so they appear to be a single shaft, although they rotate totally
    independently of each other. The input length of this shaft is much shorter than the output shaft.
    Parallel to the mainshaft is the countershaft. There are a number of gears fixed along the
    countershaft, and matching gears along the output shaft, although these are not fixed, and
    rotate independently of the output shaft. There are sliding dog collars, or dog clutches,
    between the gears on the output shaft, and to engage a gear to the shaft, the collar slides
    into the space between the shaft and the inside space of the gear, thus rotating the shaft as
    well. One collar is usually mounted between two gears, and slides both ways to engage one
    or the other gears, so on a four speed there would be two collars. A front-drive transmission
    is basically the same, but may be simplified. There often are two shafts, the input and the
    output, but depending on the direction of rotation of the engine, three may be required. Rather
    than the input shaft driving the countershaft with a pinion gear, the input shaft takes over the
    countershaft's job, and the output shaft runs parallel to it. The gears are positioned and
    engaged just as they are on the countershaft and output shaft of a rear-drive. This merely
    eliminates one major component, the pinion gear. Part of the reason that the input and
    output are in-line on a rear drive unit is to relieve torsional stress on the transmission
    and mountings, but this isn't an issue in a front-drive as the gearbox is integrated
    into the transaxle.

    The basic process is not universal. The fixed and free gears can be mounted on either the
    input or output shaft, or both.

    The distribution of the shifters is also a matter of design; it need not be the case that all of
    the free-rotating gears with selectors are on one shaft, and the permanently splined gears
    on the other. For instance a five speed transmission might have the first-to-second selectors
    on the countershaft, but the third-to-fourth selector and the fifth selector on the mainshaft,
    which is the configuration in the 1998 Honda Civic. This means that when the car is stopped
    and idling in neutral with the clutch engaged and the input shaft spinning, the third, fourth
    and fifth gear pairs do not rotate.

    In some transmission designs (Volvo 850 and V/S70 series, for example) there are actually two
    countershafts, both driving an output pinion meshing with the front-wheel-drive transaxle's
    ring gear. This allows the transmission designer to make the transmission narrower, since
    each countershaft need only be half as long as a traditional countershaft with four gears
    and two shifters.

    Clutch

    In all vehicles using a transmission (virtually all modern vehicles), a coupling
    device is used to separate the engine and transmission when necessary. The
    clutch accomplishes this in manual transmissions. Without it, the engine and
    tires would at all times be inextricably linked, and any time the vehicle stopped
    the engine would stall. Without the clutch, changing gears would be very
    difficult, even with the vehicle moving already: deselecting a gear while the
    transmission is under load requires considerable force, and selecting a gear
    requires the revolution speed of the engine to be held at a very precise
    value which depends on the vehicle speed and desired gear. In a car the
    clutch is usually operated by a pedal; on a motorcycle, a lever on the left
    handlebar serves the purpose.

• When the clutch pedal is fully depressed, the clutch is fully disengaged,
  and no torque is transferred from the engine to the transmission (and by
  extension to the drive wheels). In this uncoupled state it is possible to
  select gears or to stop the car without stopping the engine.
• When the clutch pedal is fully released, the clutch is fully engaged, and
  practically all of the engine's torque is transferred. In this coupled state,
  the clutch does not slip, but rather acts as rigid coupling, and power is
  transmitted to the wheels with minimal practical waste heat.
• Between these extremes of engagement and disengagement the clutch
  slips to varying degrees. When the clutch slips it still transmits torque
  despite the difference in speeds between the engine crankshaft and the
  transmission input. Because this torque is transmitted by means of friction
  rather than direct mechanical contact, considerable power is wasted as
  heat (which is dissipated by the clutch). Properly applied, slip allows the
  vehicle to be started from a standstill, and when it is already moving,
  allows the engine rotation to gradually adjust to a newly selected gear
  ratio.
• Learning to use the clutch efficiently requires the development of
  muscle memory and a level of coordination analogous to that required
  to learn a musical instrument or to play a sport.
• A rider of a highly tuned motocross or off-road motorcycle may
  "hit" or "fan" the clutch when exiting corners to assist the engine
  in revving to the point where it delivers the most power.

    Gear shift types

    Floor-mounted shifter

    In most vehicles with manual transmission, gears are selected by manipulating
    a lever called a gear stick, shift stick, gearshift, gear lever, gear selector,
     or shifter connected to the transmission via linkage or cables and mounted on
    the floor, dashboard, or steering column. Moving the lever forward, backward,
    left, and right into specific positions selects particular gears.

    A sample layout of a four-speed transmission is shown below. N marks neutral,
    the position wherein no gears are engaged and the engine is decoupled from
    the vehicle's drive wheels. The entire horizontal line is a neutral position,
    though the shifter is usually spring loaded so it will return to the centre of
    the N position if not moved to another gear. The R marks reverse, the
    gear position used for moving the vehicle rearward.

    This layout is called the shift pattern. Because of the shift quadrants,
    the basic arrangement is often called an H-pattern. The shift pattern is usually
    molded or printed on or near the gear knob. While the layout for gears one
    through four is nearly universal, the location of reverse is not. Depending
    on the particular transmission design, reverse may be located at the upper
    left extent of the shift pattern, at the lower left, at the lower right, or at the
    upper right. There is often a mechanism that allows selection of reverse only
    from the neutral position, or a reverse lockout that must be released by
    depressing the spring-loaded gear knob or lifting a spring-loaded collar
    on the shift stick, to reduce the likelihood of the driver inadvertently
    selecting reverse.

    Four-speed transmissions with floor-mounted shifters were sometimes
    referred to as "four on the floor" during the period when the steering column
    was the more common shifter location. The latter, often being the
    standard non-performance transmission, usually had only three forward
    speeds and was referred to as "three on the tree."

    Most front-engined, rear-wheel drive cars have a transmission that sits
    between the driver and the front passenger seat. Floor-mounted shifters
    are often connected directly to the transmission. Front-wheel drive and
    rear-engined cars often require a mechanical linkage to connect the
    shifter to the transmission.

    Column-mounted shifter

    Some cars have a gear lever mounted on the steering column of the car.
    A 3-speed column shifter, which came to be popularly known as a "Three
    on the Tree", began appearing in America in the late 1930s and became
    common during the 1940s and '50s. If the vehicle was equipped with
    overdrive, it was actuated by a control separate from the main
    3-speed-plus-reverse shift pattern shown here:

    Later, European and Japanese models began to have 4-speed
    column shifters with this shift pattern:

    The column-mounted manual shifter disappeared in North America by the
    mid 1980s, last appearing in the 1987 Chevrolet pickup truck. Outside
    North America, the column mounted shifter remained in production. All
    Toyota Crown and Nissan Cedric taxis in Hong Kong had the 4-speed
    column shift until 1999 when automatic transmissions were first offered.
    Since the late 1980s or early 1990s, a 5-speed column shifter has been
    offered in some vans sold in Asia and Europe, such as Toyota Hiace
    and Mitsubishi L400.

    Column shifters are mechanically similar to floor shifters, although shifting
    occurs in a vertical plane instead of a horizontal one. Because the shifter is
    further away from the transmission, and the movements at the shifter and
    at the transmission are in different planes, column shifters require more
    complicated linkage than floor shifters. Advantages of a column shifter
    are the ability to switch between the two most commonly used gears—second
    and third—without letting go of the steering wheel, and the lack of
    interference with passenger seating space in vehicles equipped
    with a bench seat.

    Console-mounted shifter

    Newer small cars and MPVs, like the Suzuki MR Wagon, the Fiat Multipla,
    the Toyota Matrix, the Pontiac Vibe, the Chrysler RT platform cars and
    the Honda Civic Si EP3 may feature a manual or automatic transmission
    gear shifter located on the vehicle's instrument panel. Console-mounted
    shifters are similar to floor-mounted gear shifters in that most of the ones
    used in modern cars operate on a horizontal plane and can be mounted to
    the vehicle's transmission in much the same way a floor-mounted shifter
    can. However, because of the location of the gear shifter in comparison to
    the locations of the column shifter and the floor shifter, as well as the
    positioning of the shifter to the rest of the controls on the panel often
    require that the gearshift be mounted in a space that does not feature
    a lot of controls integral to the vehicle's operation or frequently used
    controls, such as those for the car stereo or car air conditioning, to
    help prevent accidental activation or driver confusion, especially in
    right-hand drive cars.

    More and more small cars and vans from manufacturers such as Suzuki,
    Honda, and Volkswagen are featuring console shifters in that they free up
    space on the floor for other car features such as storage compartments
    without requiring that the gear shift be mounted on the steering column.
    Also, the basic location of the gear shift in comparison to the column
    shifter makes console shifters easier to operate than column shifters.

    Sequential manual

    Some transmissions do not allow the driver to arbitrarily select any gear.
    Instead, the driver may only ever select the next-lowest or next-highest
    gear ratio. Sequential transmissions often incorporate a synchro-less dog-clutch
    engagement mechanism (instead of the synchromesh dog clutch common
    on H-pattern automotive transmissions), in which case the clutch is only
    necessary when selecting first or reverse gear from neutral, and most gear
    changes can be performed without the clutch. However, sequential shifting
    and synchro-less engagement are not inherently linked, though they often
    occur together due to the environment(s) in which these transmissions are
    used, such as racing cars and motorcycles.

    Sequential transmissions are generally controlled by a forward-backward
    lever, foot pedal, or set of paddles mounted behind the steering wheel.
    In some cases, these are connected mechanically to the transmission. In
    many modern examples, these controls are attached to sensors which
    instruct a transmission computer to perform a shift—many of these systems
    can be switched into an automatic mode, where the computer controls the
    timing of shifts, much like an automatic transmission.

    Motorcycles typically employ sequential transmissions, although the shift
    pattern is modified slightly for safety reasons. In a motorcycle the gears
    are usually shifted with the left foot pedal, the layout being this:

    6 - 5 - 4 - 3 - 2 N 1

    The pedal goes one step–both up and down–from the center, before it
    reaches its limit and has to be allowed to move back to the center position.
    Thus, changing multiple gears in one direction is accomplished by
    repeatedly pumping the pedal, either up or down. Although neutral is
    listed as being between first and second gears for this type of transmission,
    it "feels" more like first and second gear are just "further away" from each
    other than any other two sequential gears. Because this can lead to difficulty
    in finding neutral for inexperienced riders most motorcycles have a neutral
    indicator light on the instrument panel to help find neutral. The reason
    neutral does not actually have its own spot in the sequence is to make it
    quicker to shift from first to second when moving. Neutral can be accidentally
    shifted into, though most high end, newer model motorcycles have means
    of avoiding this. The reason for having neutral between the first and second
    gears instead of at the bottom is that when stopped, the rider can just click
    down repeatedly and know that they will end up in first and not neutral.
    This allows a rider to quickly move his bike from a standstill in an
    emergency situation. This may also help on a steep hill on which high
    torque is required. It could be disadvantageous or even dangerous
    to attempt to be in first without realizing it, then try for a lower gear,
    only to get neutral.

    On motorcycles used on race tracks, the shifting pattern is often reversed,
    that is, the rider clicks down to upshift. This usage pattern increases the
    ground clearance by placing the riders foot above the shift lever when the
    rider is most likely to need it, namely when leaning over and exiting
    a tight turn.

    The shift pattern for most underbone motorcycles with an automatic
    centrifugal clutch is also modified for two key reasons - to enable the
    less-experienced riders to shift the gears without problems of "finding"
    neutral, and also due to the greater force needed to "lift" the gearshift
    lever (because the gearshift pedal of an underbone motorcycle also
    operates the clutch). The gearshift lever of an underbone motorcycle has
    two ends. The rider clicks down the front end with the left toe all the way
    to the top gear and clicks down the rear end with the heel all the way down
    to neutral. Some underbone models such as the Honda Wave have a "rotary"
    shift pattern, which means that the rider can shift directly to neutral
    from the top gear, but for safety reasons this is only possible when the
    motorcycle is stationary. Some models also have gear position indicators
    for all gear positions at the instrument panel.

    Semi-manual

    Some new transmissions (Alfa Romeo's Selespeed gearbox and BMW's
    Sequential Manual Gearbox (SMG) for example) are conventional manual
    transmissions with a computerized control mechanism. These transmissions
    feature independently selectable gears but do not have a clutch pedal.
    Instead, the transmission computer controls a servo which disengages
    the clutch when necessary.

    These transmissions vary from sequential transmissions in that they still
    allow nonsequential shifts: BMWs SMG system, for example, can shift
    from 6th gear directly to 4th gear.

    In the case of the early second generation Saab 900, a 'Sensonic' option
    was available where gears were shifted with a conventional shifter, but
    the clutch is controlled by a computer.

    Benefits

    Fuel economy

    The manual transmission couples the engine to the transmission with a
    rigid clutch instead of the torque converter on an automatic transmission or
    the v-belt of a continuously variable transmission,[6] which slip by nature.
    Manual transmissions also lack the parasitic power consumption of the
    automatic transmission's hydraulic pump. Because of this, manual
    transmissions generally offer better fuel economy than automatic or
    continuously variable transmissions; however the disparity has been
    somewhat offset with the introduction of locking torque converters on
    automatic transmissions.[7] Increased fuel economy with a properly
    operated manual transmission vehicle versus an equivalent automatic
    transmission vehicle can range from 5% to about 15% depending on
    driving conditions and style of driving.[8] The lack of control over
    downshifting under load in an automatic transmission, coupled with
    a typical vehicle engine's greater efficiency under higher load, can
    enable additional fuel gains from a manual transmission by allowing
    the operator to keep the engine performing under a more efficient
    load/RPM combination. Also, manual transmissions do not require active
    cooling and because they are, mechanically, much simpler than automatic
    transmissions, they generally weigh less than comparable automatics,
    which can improve economy in stop-and-go traffic.[7]

    Longevity and cost

    Because manual transmissions are mechanically simpler and have fewer
    moving parts than automatic transmissions, they require less maintenance
    and are easier to repair. The price of a new car with a manual transmission
    will often be lower than the same car with an automatic transmission.

    Performance and control

    Manual transmissions generally offer a higher selection of gear
    ratios. Many vehicles offer a 5-speed or 6-speed manual, whereas
    the automatic option would typically be a 4-speed. This is generally
    due to the space available inside of a manual transmission versus an
    automatic since the latter requires extra components for self-shifting,
    such as torque converters and pumps. However, automatic
    transmissions are now adding more speeds as the technology
    matures. ZF currently makes 7 and 8-speed automatic transmissions,
    the higher selection of gears allows for more use of the engine's
    power band, allowing either better fuel economy, by staying in the
    most fuel efficient part of the power band, or higher power output,
    by staying closer to the engine's peak power.

    Currently, only fully manual transmissions allow the driver to
    fully exploit the engine power at low to medium engine speeds.
    This is because even automatic transmissions which provide some
    manual mode (e.g. tiptronic), use a throttle kickdown switch, which
    forces a downshift on full throttle and causes the gearbox to ignore
    a user command to upshift on full throttle. This is especially notable
    on uphill roads, where cars with automatic transmission need to slow
    down to avoid downshifts, whereas cars with manual transmission
    and identical or lower engine power are still able to maintain their
    speed. The manual transmission can also be put into a lower gear
    ahead of time, to make rapid acceleration, such as when overtaking
    on the highway, more instantaneous.

    Because the driver has more direct control over the car with a manual
    than with an automatic, an experienced, knowledgeable driver who
    knows the correct procedure for executing a driving maneuver, and
    wants the vehicle to realize his or her intentions exactly and instantly,
    can perform actions difficult or impossible with automatic
    transmissions. When starting forward, for example, the driver
    can control how much torque goes to the tires, which is useful
    on slippery surfaces such as ice, snow or mud. This can be done
    with clutch finesse, or by starting in second gear instead of first.
    However, some automatic transmissions, particularly those with
    tiptronic gear selectors, can be placed in second gear for starting
    on slippery surfaces. An engine coupled with a manual transmission
    can often be started by the method of push starting. This is particularly
    useful if the starter is inoperable or defunct, or the battery has drained
    below operable voltage. Similarly, a vehicle with a manual transmission
    and no clutch/starter interlock switch can be moved, if necessary, by
    cranking the starter while in gear. This is useful when the vehicle
    will not start, but must be immediately moved, e.g., off the road in
    the event of a breakdown, if the vehicle has stalled on a railway
    crossing, or in extreme off-roading cases such as an engine that
    has stalled in deep water. Because the torque converter in an automatic
    transmission is viscous by nature, the low speeds gained by
    push-starting are generally unable to produce enough torque
    on the crankshaft to make the engine start. Conversely, the same
    thing prevents the torque at the starter motor from reaching the
    wheels in sufficient amounts to move the car under most circumstances.

    Engine braking

    In contrast to most manual gearboxes, most automatic transmissions have
    a free-wheel-clutch. This means that the engine does not slow down the
    car when the driver steps off the throttle, also known as engine or
    compression braking. This leads to more usage of the brakes in cars
    with automatic transmissions, which can potentially overheat them in
    hilly or mountainous areas, causing reduced braking ability and the
    potential for complete failure. However, the automatic gearboxes in
    commodity Nissans and Hondas disable the free wheel operation
    completely if the driver has selected a gear position other than "D"
    - either "1", "2", or "D with overdrive off". This works by blocking
    the free-wheel sprag using a multi-disk clutch called the "overrun
    clutch". Similarly, many trucks have a "tow mode" that allows
    engine braking.

    Drawbacks

    Complexity and learning curve

    There is a significant learning curve with a manual transmission, and
    the smoothness and correct timing of gear shifts are wholly dependent
    on the driver's experience and skill. Because the driver must develop a
    feel for properly engaging the clutch, especially when starting forward
    on a steep road or when parking on an incline, an inexperienced driver
    can easily stall the engine, or cause the car to jerk and bounce abruptly,
    which is not only uncomfortable, but could potentially be dangerous in
    an area with little room for error. Additionally, if an inexperienced driver
    selects the wrong gear by mistake, they can do damage to the engine
    and/or transmission. Selecting too high a gear can "lug" the motor,
    lowering the lifespan over time, while selecting too low a gear for the
    speed of the car can over-rev the engine, causing severe damage
    very quickly.

    Attempting to select reverse while the vehicle is moving forward causes
    severe gear wear (except in transmissions with synchromesh on the
    reverse gear). However, most manual transmissions have a gate that
    locks out reverse directly from 5th gear to help prevent this. In order
    to engage reverse from 5th, the shift lever has to be moved to the
    center position between 2nd and 3rd, then back over and into reverse.
    Similarly, many newer six-speed manual transmissions have a collar
    under the shift knob which must be lifted to engage reverse to also
    help prevent this.

    Shifting speed

    Some automatic transmissions can shift ratios faster than a manual gear
    change can be accomplished, due to the time required for the average driver
    to push the clutch pedal to the floor and move the gearstick from one
    position to another. This is especially true in regards to dual clutch
    transmissions, which are specialized computer-controlled manual
    transmissions. Even though some automatic transmissions and
    semi-automatic transmissions can shift faster, many purists still
    prefer a regular manual transmission.

    Ease of use

    Because manual transmissions require the operation of an extra pedal,
    and keeping the car in the correct gear at all times, they require a bit
    more concentration, especially in heavy traffic situations. The automatic
    transmissions, on the other hand, simply require the driver to speed up
    or slow down as needed, with the car doing the work of choosing the
    correct gear. Manual transmissions also place a greater workload on the
    driver in heavy traffic situations, when the driver must operate the
    clutch pedal quite often. Because the clutch pedal can require a
    substantial amount of force, especially on large trucks, and the long
    pedal travel compared to the brake or accelerator requires moving the
    entire leg, not just the foot near the ankle, a manual transmission can
    cause fatigue, and is more difficult for weak or injured people to drive.
    Additionally, because automatic transmissions can be driven with only
    one foot, people with one leg that is missing or impaired can still drive,
    unlike the manual transmission that requires the use of two feet at once.
    Likewise, manual transmissions require the driver to remove one hand
    periodically from the steering wheel while the vehicle is in motion, which
    can be difficult or impossible to do safely for people with a missing or
    impaired arm, and requires increased coordination, even for those with
    full use of both hands.

    Stopping on hills

    Because of manual transmissions requirement of clutch-throttle balance
    difficulty, when stopped on a hill, causes the car to roll and wheel spin
    and requires smooth shifting to make up a steep hill.

    Applications and popularity

    Many types of automobiles are equipped with manual transmissions. Small
    economy cars predominantly feature manual transmissions because they are
    cheap and efficient, although many are optionally equipped with automatics.
    Economy cars are also often powered by very small engines, and manual
    transmissions make more efficient use of the power produced.

    Sports cars are also often equipped with manual transmissions because they
    offer more direct driver involvement and better performance. Off-road
    vehicles and trucks often feature manual transmissions because they allow
    direct gear selection and are often more rugged than their automatic counterparts.

    Conversely, manual transmissions are no longer popular in many classes of
    cars sold in North America, Australia and some parts of Asia, although they
    remain dominant in Europe, Asia and developing countries. Nearly all cars
    are available with an automatic transmission option, and family cars and
    large trucks sold in the US are predominantly fitted with automatics, however
    in some cases if a buyer wishes he/she can have the car fitted with a manual
    transmission at the factory. In Europe most cars are sold with manual
    transmissions. Most luxury cars are only available with an automatic
    transmission. In most cases where both transmissions are available for a
    given car, automatics are an at cost option, but in some cases the reverse
    is true. Some cars, such as rental cars and taxis, are nearly universally
    equipped with automatic transmissions in countries such as the US, but
    the opposite is true in Europe.[9] As of 2008, 75.2% of vehicles made
    in Western Europe were equipped with manual transmission, versus
    16.1% with automatic and 8.7% with other.[10]

    In some places (for example New Zealand (for the second-phase Restricted
    licence, but not the final Full licence), Belgium, China, Estonia, Dominican
    Republic, Finland, France, Germany, Ireland, Israel, Jordan, Netherlands,
    Norway, Poland, Singapore, Slovenia, South Africa, South Korea, Spain,
    Sri Lanka, Sweden, Turkey, U.A.E and the UK), when a driver takes the
    licensing road test using an automatic transmission, the resulting license
    is restricted to the use of automatic transmissions. This treatment of the
    manual transmission skill seems to maintain the widespread use of the
    manual transmission. As many new drivers worry that their restricted
     license will become an obstacle for them where most cars have manual
    transmissions, they make the effort to learn with manual transmissions
    and obtain full licenses. Some other countries (such as India, Pakistan,
    Malaysia, Serbia, Brazil, Russia, Ukraine and Denmark) go even further,
    whereby the license is granted only when a test is passed on a manual
    transmission. In Denmark and Brazil drivers are allowed to take the test
    on an automatic if they are handicapped, but with such a license they will
    not be allowed to drive a car with a manual transmission.

    Truck transmissions

    Some trucks have transmissions that look and
    behave like ordinary car transmissions - these
    transmissions are used on lighter trucks,
    typically have up to 6 gears, and usually
    have synchromesh.

    For trucks needing more gears, the standard
    "H" pattern can get very complicated, so
    additional controls are used to select additional
    gears. The "H" pattern is retained, then an
    additional control selects among alternatives.
    In older trucks, the control is often a separate
    lever mounted on the floor or more recently
    a pneumatic switch mounted on the "H" lever;
    in newer trucks the control is often an electrical
    switch mounted on the "H" lever. Multi-control
    transmissions are built in much higher power
    ratings, but rarely use synchromesh.

    There are several common alternatives for the
    shifting pattern. Usual types are:

• Range transmissions use an
  "H" pattern through a narrow
  range of gears, then a "range"
  control shifts the "H" pattern
  between high and low ranges.
  For example, an 8-speed range
  transmission has an H shift
  pattern with four gears. The
  first through fourth gears are accessed when low range is selected. To access the fifth through eighth gears, the
  range selector is moved to
  high range, and the gear
  lever again shifted through
  the first through fourth gear
  positions. In high range, the
  first gear position becomes
  fifth, the second gear position
  becomes sixth, and so on.

• Splitter transmissions use an
  "H" pattern with a wide range
  of gears, and the other selector
  splits each sequential gear position in two: First gear is
  in first position/low split,
  second gear is in first position/high split, third
  gear is in second position/low split, fourth gear is in second
  position/high split, and so on.

• Range-Splitter transmissions combine range-splitting and gear-splitting. This allows
  even more gear ratios. Both
  a range selector and a
  splitter selector are provided.

    Although there are many gear positions, shifting
    through gears usually follows a regular pattern.
    For example, a series of upshifts might use "move
    to splitter direct; move to splitter overdrive; move
    shift lever to #2 and move splitter to underdrive;
    move splitter to direct; move splitter to overdrive;
    move shift lever to #3 and move splitter to
    underdrive"; and so on. In older trucks using
    floor-mounted levers, a bigger problem is
    common gear shifts require the drivers to move
    their hands between shift levers in a single shift,
    and without synchromesh, shifts must be carefully
    timed or the transmission will not engage. For this
    reason, some splitter transmissions have an
    additional "under under" range, so when the
    splitter is already in "under" it can be quickly
    downshifted again, without the delay of a
    double shift.

    Today's truck transmissions are most commonly
    "range-splitter". The most common 13 speed has
    a standard H pattern, and the pattern from left
    upper corner is as follows: R, down to L, over and
    up to 1, down to 2, up and over to 3, down to 4.
    The "butterfly" range lever in the center front of
    the knob is flipped up to high range while in 4th,
    then shifted back to 1. The 1 through 4 positions
    of the knob are repeated. Also, each can be split
    using the thumb-actuated under-overdrive lever
    on the left side of the knob while in high range.
    The "thumb" lever is not available in low range,
    except in 18 speeds; 1 through 4 in low range
    can be split using the thumb lever and L can be
    split with the "Butterfly" lever. L cannot be split
    using the thumb lever in either the 13 or 18 speed.
    The 9 speed transmission is basically a 13 speed
    without the under-overdrive thumb lever.

    Truck transmissions use many physical layouts. For
    example, the output of an N-speed transmission
    may drive an M-speed secondary transmission,
    giving a total of N*M gear combinations; for
    example a 4-speed main box and 3-speed splitter
    gives 12 ratios. Transmissions may be in separate
    cases with a shaft in between; in separate cases
    bolted together; or all in one case, using the same
    lubricating oil. The second transmission is often
    called a "Brownie" or "Brownie box" after a popular
    brand. With a third transmission, gears are
    multiplied yet again, giving greater range or
    closer spacing. Some trucks thus have dozens
    of gear positions, although most are duplicates.
    Sometimes a secondary transmission is integrated
    with the differential in the rear axle, called a
    "two-speed rear end." Two-speed differentials
    are always splitters. In newer transmissions,
    there may be two countershafts, so each main
    shaft gear can be driven from one or the other
    countershaft; this allows construction with
    short and robust countershafts, while still
    allowing many gear combinations inside a
    single gear case.

    Heavy-duty transmissions are almost always
    non-synchromesh. One argument is
    synchromesh adds weight that could be
    payload, is one more thing to fail, and drivers
    spend thousands of hours driving so can take
    the time to learn to drive efficiently with a
    non-synchromesh transmission. Heavy-duty
    trucks driven frequently in city traffic, such
    as cement mixers, need to be shifted very
    often and in stop-and-go traffic. Since few
    heavy-duty transmissions have synchromesh,
    automatic transmissions are commonly used
    instead, despite their increased weight, cost,
    and loss of efficiency.

    Heavy trucks are usually powered with diesel
    engines. Diesel truck engines from the 1970s
    and earlier tend to have a narrow power band,
    so need many close-spaced gears. Starting with
    the 1968 Maxidyne, diesel truck engines have
    increasingly used turbochargers and electronic
    controls that widen the power band, allowing
    fewer and fewer gear ratios. A transmission
    with fewer ratios is lighter and may be more
    efficient due to fewer transmissions in series.
    Fewer shifts also makes the truck more drivable.
    As of 2005, fleet operators often use 9,10,13
    or 18-speed transmissions, but automated
    manual and semi-automatic transmissions
    are becoming more common on heavy vehicles,
    as they can improve efficiency and drivability,
    reduce the barrier to entry for new drivers,
    and may improve safety by allowing the
    driver to concentrate on road conditions.

    Maintenance

    Because clutches use changes in friction to modulate the transfer of torque
    between engine and transmission, they are subject to wear in everyday use.
    A very good clutch, when used by an expert driver, can last hundreds of
    thousands of kilometres (or miles). Weak clutches, abrupt downshifting,
    inexperienced drivers, and aggressive driving can lead to more frequent
    repair or replacement.

    Manual transmissions are lubricated with gear oil or engine oil in some
    cars, which must be changed periodically in some cars, although not as
    frequently as the automatic transmission fluid in a vehicle so equipped.
    (Some manufacturers specify that changing the gear oil is never necessary
    except after transmission work or to rectify a leak.)

    Gear oil has a characteristic aroma due to the addition of sulfur-bearing
    anti-wear compounds. These compounds are used to reduce the high
    sliding friction by the helical gear cut of the teeth (this cut eliminates
    the characteristic whine of straight cut spur gears). On motorcycles with
    "wet" clutches (clutch is bathed in engine oil), there is usually nothing
    separating the lower part of the engine from the transmission, so the
    same oil lubricates both the engine and transmission. The original
    Mini placed the gearbox in the oil sump below the engine, thus using
    the same oil for both.

• 

  Reverse

• 

  Neutral

• 

  First gear

• 

  Second gear

• 

  Third gear
  
  

      Referenced information from http://en.wikipedia.org