Overdrive can refer to two different things. An overdrive is a device which was commonly used in automobiles to allow the choice of an extra-high overall gear ratio for high speed cruising, thus saving fuel, at the cost of less torque. Usually the final or top gear is called overdrive. Non direct drive gears increase torque multiplication at the expense of higher engine rpm and thus, decreased fuel economy.
It also refers to a combination of gearing inside a transmission or transaxle which results in the output speed being greater than the input speed. In the latter case, "overdrive" does not refer to a physically separate identifiable part/assembly.
