Need for a gearbox
Automotive transmissions - Role and determination of gear ratios

An automotive gearbox is a mechanical device that adapts the torque and speed of the engine to the torque and speed required by the axle, depending on the driving conditions encountered.

Although continuously-variable transmissions exist, most gearboxes have discrete ratios, with the various gear ratios provided by helical gears with parallel axes [B 5 660] or epicyclic gears. Figure 1 shows an example of a six-speed manual gearbox, with details of the kinematics used...