The drivetrain of a motor vehicle is the group of parts that deliver power to the wheels that move the vehicle. The drivetrain includes the transmission or transaxle, driveshafts, differentials, axles and wheels. It does not include the engine or motor that creates the power. In contrast, the powertrain includes the engine and the drivetrain. In order to transmit power from the engine to the rear wheels, the drivetrain uses some of that power to work. Drivetrain loss is the power lost, measured in horsepower, between what the engine generates and what is available at the wheels. As power is transmitted through the drivetrain, forces including inertia, drag and friction cause some power to be wasted. The more component parts power is transmitted through, the more power is lost. However each combination of drivetrain parts is different as to how much is lost. The automatic transmission wastes the most power of any drivetrain component.