The internal combustion engines generally installed in motor vehicles develop their power output at high speeds of rotation (approx. 4000-6000 r.p.m.).

It is therefore necessary to reduce the speed between the crankshaft and the shaft which drives the wheels. In addition, the torque (see below) that the engine delivers can be varied only within narrow limits.

For this reason it is necessary to be able to alter the transmission ratio, so that the driving forces applied to the wheels of the vehicle can be adapted to the varying road resistance conditions.

Part of the requisite total speed reduction is effected in the final drive (see differential gear). The rest of the reduction can be obtained by means of the change-speed gearbox, which is interposed between the clutch and the final drive. In addition, the gearbox contains the mechanism for reversing. The gearbox alters the torque that is transmitted and therefore functions as a so-called torque converter. The transmission ratio (or gear ratio) is dependent upon the ratio of the numbers of teeth of the meshing gear wheels: a gear wheel mounted on a shaft rotating at a certain speed will drive another shaft whose gear wheel has half the number of teeth at twice the speed of rotation of the first shaft. Since the forces acting upon the two gear wheels are equal, the torque - i.e., the product of the force acting tangentially upon the gear wheel and the distance from the point of application of this force to the centre of rotation of the shaft - will, for the larger gear wheel, be double that for the small one.

On the other hand, the smaller wheel will revolve at twice the speed of the larger. In other words, the smaller gear wheel has the higher speed, but the lower torque; the larger wheel rotates more slowly, but transmits the higher torque. The transmission ratio is the ratio of the input speed to the output speed; the pitch circle diameters of the gear wheels and torques of the shafts are in the inverse ratio.