In normal gear conditions, the engine of a car turns at 3,500rpm; if we make wheels spin at this speed, the car will run at a speed faster than 350km/h. Therefore, it is clear that there must be an intermediate element to modify the transmission relationship between the engine and the wheels, so that we can always get maximum performance from the engine. This element will also have to allow the direction of the vehicle's gearing to be reversed without reversing the engine's spinning, as this is not possible.