A gearbox is an assembly of gears allowing the rotational speed of an input shaft to be changed to a different speed. Often, more than one gear ratio is provided, and a mechanism for selecting among the ratios is provided. The most common form of gearbox encountered in everyday life is that of a car with a manual transmission.

A modern car gearbox is of the constant mesh or synchromesh type, in which all gears are in mesh, but only one of which is locked to the shaft on which it is mounted at any one time, the others being allowed to rotate freely. This type of gearbox avoids the problems of wear and damage caused by bringing moving gears into mesh, however, it has higher frictional losses than the simpler type.

Heavy trucks and machinery usually are fitted with a simpler type of gearbox, known colloquially as a "crash gearbox". Here, the selected gear is brought directly into mesh, and any others are not involved. Driving a vehicle with such a gearbox requires considerable skill, including mastering the technique known as double declutching.

Synchromesh

In a synchromesh gearbox, gears can freely rotate or be locked to the shaft on which they are carried. The locking mechanism consists of a sliding collar which bridges between two circular rings with teeth on them - one travels with the gear, one with the shaft. When the rings are bridged, the gear is locked to the shaft. 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 clutch which is attached to the gear. This spins the gear up or down in speed to match the shaft prior to engagement of the collar. The collar is prevented from bridging the locking rings when the speeds are mismatched by baulk rings. The gear lever manipulates the collars using a set of linkages, so arranged so that only one collar may be permitted to lock a gear at any one time. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed.