A car uses overdrive when it is traveling at high speed over long distances. Overdrive, or cruising gear, is a device which enables the engine to run at a relatively low speed even when the vehicle is traveling fast.
All internal combustion engines fitted in vehicles need some kind of gearbox because their efficiency at low speeds is poor. The use of different gears enables the speed of the engine to be harmonized with that of the car. The gears may be engaged or shifted by hand or operated automatic gearbox.
Most cars have a four-speed gearbox. The driver used first gear for starting and changes to second and third gears as the car gains speed. Finally in top or fourth gear the engine speed is transmitted unreduced through the gearbox. In overdrive a large gear wheel drives a smaller gear wheel on the propeller shaft. This shaft then rotates faster than the engine, thus reducing wear and tear and saving petrol.