<P> An overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behind the transmission unit . It can either couple the input driveshaft directly to the output shaft (or propeller shaft) (1: 1), or increase the output speed so that it turns faster than the input shaft (1: 1 + n). Thus the output shaft may be "overdriven" relative to the input shaft . In newer transmissions, the overdrive speed (s) are typically as a result of combinations of planetary / epicyclic gearsets which are integrated in the transmission . In these cases, there is no separately identifiable "overdrive" unit . In older vehicles, it is sometimes actuated by a knob or button, often incorporated into the gearshift knob, and does not require operation of the clutch . Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load . </P> <P> The vast majority of overdrives in European cars were invented and developed by a man called de Normanville and manufactured by an English company called Laycock Engineering (later GKN Laycock), at its Little London Road site in Sheffield . The system was devised by Captain Edgar J de Normanville (1882--1968), and made by Laycock through a chance meeting with a Laycock Products Engineer . De Normanville overdrives were found in vehicles manufactured by Standard - Triumph, who were first, followed by Ford, BMC and British Leyland, Jaguar, Rootes Group and Volvo to name only a few . Another British company, the former aircraft builder Fairey, built a successful all - mechanical unit for the Land Rover, which is still in production in America today . </P> <P> The first production vehicle to feature the Laycock system was the 1948 Standard Vanguard Saloon . The first unit to be created was the A-type overdrive, which was fitted to many sports cars during the 1950s, and into the late 1960s . Several famous marques used A-type overdrives, including Jaguar, Aston Martin, Ferrari, Austin - Healey, Jensen, Bristol, AC, Armstrong Siddeley and Triumph's TR sports car range, from the TR2 through to the end of the 1972 model year of the TR6 . </P> <P> In 1959, the Laycock Engineering Company introduced the D - type overdrive, which was fitted to a variety of motor cars including Volvo 120 and 1800s, Sunbeam Alpines and Rapiers, Triumph Spitfires, and also 1962 - 1967 MGBs (those with 3 - synchro transmissions). </P>

When do you use overdrive in an automatic