Overdrives

To overdrive means to drive faster. In a vehicle transmission, the propeller shaft drives faster than the engine when an overdrive gear is in use. This can be a fifth gear in the gearbox as on modern cars and later Land Rovers or in the case of Series Land Rovers it is an optional bolt-on unit that fits on the rear of the transfer case.

The most common type found on series Land Rovers is the Fairey, later made under the Superwinch name. Toro is another manufacturer but is far less common.

Fairey/Superwinch overdrives

Parts diagrams and lists 
Fitting instructions