A preselector gearbox is a type of gearbox used on a variety of vehicles, more commonly until around the 1950s. First used in World War I tanks, it was originally patented in 1928 by Major Walter Gordon Wilson (1874–1957). Various manufacturers produced preselector transmissions under licence to the Wilson patents. One of the top manufacturers was a French company called Cotal.
As its name suggests, gear changes were made by selecting a gear ratio in advance of its being needed. The chosen gear was then brought into operation by pressing and releasing the 'gear change pedal', which was normally the left pedal, installed in place of the usual clutch pedal. It is not to be confused with automatic transmission, in that gear changing is initiated by the driver. Unlike the "crash" gearboxes of the first half of the 20th century, the gearwheels in a preselector box are permanently in mesh in an epicyclic layout.
They were common on Daimler, Alvis, and Armstrong Siddeley cars as well as on many London buses. They have also been used in racing cars, such as the 1935 ERA R4D , and hillclimbing cars such as Auto Union "Silver Arrows". Military applications included tanks such as the German Tiger I and Tiger II in World War II, through to the current tanks such as Challenger 2.
On some cars, starting off from rest involved using the gear change pedal like a clutch. On others, first gear could be selected but while the engine was still idling the car would not move even after the gear change pedal had been pressed and released. When the accelerator was pressed a centrifugal clutch or fluid coupling would engage and the car would begin to move.