The ratio between the driveshaft or transmission output shaft rpm and the drive-wheel axle shaft rpm. For example, if the ratio is 4.00:1 the driveshaft rotates four times for each rotation of the rear axle differential gear, the axle shafts and the wheels.

The reduction ratio, found in the gearset of a drivetrain, that is furthest removed from the engine. Typically, the differential ratio.

A fluid coupling consists of two fan-like impellers in a sealed, oil-filled housing. The input "fan" churns the oil, and the churning oil, in turn, twirls the output "fan." Such a coupling allows some speed difference between its input and output shafts. The automatic transmission's torque converter is based on the fluid coupling principle.