An overdrive is a device which was commonly used on rear-wheel drive automobiles to allow the choice of an extra-high overall gear ratio for high speed cruising, thus saving fuel.
History
Automotive transmissions manufactured since the 1980s tend to include a larger selection of gear ratios than before, the highest of which is usually greater than 1:1. This trend has rendered overdrives a complex and obsolete solution to economy gearing in automobiles, and very few cars are fitted with them today.
How overdrive works
The overdrive consists of a hydraulically operated epicyclic gear train bolted behind the transmission unit. It can either transfer the input drive shaft directly to the output shaft, called a propshaft (1:1), or increase the propshaft speed so that it turns faster than the input shaft (1:1 + n). Thus the propshaft may be "overdriven" relative to the input shaft.
Overdrive in Europe
The vast majority of overdrives in European cars were manufactured by an English company called Laycock de Normanville, which is now defunct, but its overdrives were found in vehicles manufactured by Ford, British Leyland, Jaguar, Rootes and Volvo to name but a few.
See also
Freewheel
