A device that connects and disconnects the wheels from the engine enabling the gears to be changed.

This is the end of the drive train before power is transmitted to the wheels. In a typical car, the engine (or electric motor) transmits its power through some sort of clutch into a transmission. Then the power is transmitted to differential gears that adjust the engine speed to the most efficient for the use intended. These final drive differential gears are either at the front axle or rear axle, depending on the vehicle's layout. A typical family car or one intended for high speeds will have a low numerical ratio, to give it speed and good fuel efficiency. A truck or performance car is likely to have a high numerical ratio for better pulling power or for better acceleration.

Most early cars used a rear axle that was one unit, solidly linking both rear wheels, the differential and brake assemblies. Sports car competition showed that if the wheel assemblies can be made lighter, including brakes, hubs and axle parts, the car will ride and handle better. As a result, manufacturers have unlinked the wheel assemblies using jointed axles so the wheels can respond to bumps separately, thus giving a better ride and also more grip to the road.