The drivetrain of a vehicle allows efficient transmission of rotational force from the engine to the wheel and hence the ground. The drive system consists of a series of gears, driveshafts, and differential units to transmit power to the wheels. Most systems consist of a transmission, a driveshaft, and a differential unit contained within an axle. Four wheel drive systems add another differential, driveshaft, and a transfer case between the transmission and the driveshafts.