A manual transmission (also known as a stick shift, straight drive, or standard transmission) is a type of transmission used in automotive applications. Manual transmissions often feature a driver-operated clutch and a movable gear selector. Most automobile manual transmissions allow the driver to select any 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-highest or next-lowest gear ratio. This second type of transmission is sometimes called a sequential (manual) transmission.

Manual transmissions are characterized by gear ratios that are selectable by engaging pairs of gears inside the transmission. Conversely, 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 semi-automatic transmissions.

Contemporary automotive manual transmissions are generally available with four to six forward gears and one reverse gear, although manual transmissions have been built with as few as two and as many as eight gears. Semi-trailers have at least 13 gears and as many as 24. 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.

Other types of transmission in mainstream automotive use are the automatic transmission, semi-automatic transmission, and the continuously variable transmission.

Manual transmissions come in two basic types: simple unsynchronized systems, where gears are spinning freely and their relative speeds must be synchronized by the operator to avoid noisy and damaging "clashing" and "grinding" when trying to mesh the rotating teeth; and synchronized systems, which eliminate this necessity while changing gears.

Unsynchronized transmission

The earliest automotive transmissions were entirely mechanical unsynchronized gearing systems. They could be shifted, with multiple gear ratios available to the operator, and even had reverse. However, the gears were engaged by sliding mechanisms or simple clutches, which required 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.

When upshifting, the speed of the gear driven by the engine had to drop to match the speed of the next gear; as this happened naturally when the clutch was depressed or disengaged, it was just a matter of skill and experience to hear and feel when the gears managed to mesh. However, when downshifting, the gear driven by the engine had to be sped up to mesh with the output gear, requiring letting the clutch up (engagement) for the engine to speed up the gears. Double declutching, that is, shifting once to neutral to speed up the gears and again to the lower gear, is sometimes needed. In fact, such transmissions are often easier to shift from without using the clutch at all. The clutch, in these cases, is only used for starting from a standstill. This procedure is common in racing vehicles and most production motorcycles.

Even though automotive transmissions are now almost universally synchronised, heavy trucks and machinery as well as dedicated racing transmissions are usually non-synchromesh transmissions, known colloquially as "crashboxes", 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.

Similarly, most modern motorcycles still utilize unsynchronised transmissions as synchronisers 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 selector method on modern motorcycles (pedal operated) is not conducive to having the long shift time of a synchronised gearbox. Because of this, it is still necessary to synchronise gear speeds by 'blipping-the-throttle' when shifting into a lower gear on a motorcycle.

Synchronized transmission

Top and side view of a typical manual transmission, in this case a Ford "Toploader", used in cars with external floor shifters. Modern gearboxes are constant mesh, i.e. all gears are always in mesh. Only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time, while the others are allowed to rotate freely. Thus, it greatly reduces the skill required to shift gears.

Most modern cars are fitted with a synchronized gear box, although it is entirely possible to construct a constant mesh gearbox without a synchromesh, as found in a motorcycle, for example. In a constant mesh gearbox, the transmission gears are always in mesh and rotating, but the gears are not rigidly connected to the shafts on which they rotate. Instead, the gears can freely rotate or be locked to the shaft on which they are carried. The locking mechanism for any individual 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 (one collar typically serves for two gears; sliding in one direction selects one transmission speed, in the other direction selects the other). 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. 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 which is 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 balk rings, the latter being spelled "baulk" in the UK). 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 and that of another engaged. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed.

The first synchronized transmission system was introduced by Cadillac in 1929. The modern cone system was developed by Porsche and introduced in the 1952 Porsche 356; cone synchronizers 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, five speeds, six speeds and so on became universal by the 1960s. Reverse gear, however, is usually not synchromesh, as there is only one reverse gear in the normal automotive transmission and changing gears in reverse is not required.

Design Variations
Gear Variety
Manual transmissions are often equipped with 4, 5, or 6 forward gears. Nearly all have one reverse gear. 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. Older cars were generally equipped with 3-speed transmissions, 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). Today, mass market automotive manual transmissions are essentially all 5-speeds, with 6-speed transmissions beginning to emerge in high performance vehicles in the early 1990s, and recently beginning to be offered on some high-efficiency and conventional passenger cars. A very small number of 7-speed 'manual derived' transmissions are offered on extremely high-end performance cars (supercars), such as the Bugatti Veyron 16.4, or the BMW M5. Both of these cars feature a "Paddle Shifter".

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
The input shaft need not turn a pinion which rotates the countershaft. Another possibility is that gears are mounted on the input shaft itself, meshed with gears on the countershaft, in which case the countershaft then turns the output shaft. In other words, it's a matter of design on which shaft the driven and driving gears reside.
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 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 must be only half as long as a traditional countershaft with four gears and two shifters.