When does a car use overdrive?
A car uses overdrive when it is travelling 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 travelling fast. The use of different gears enables the speed of the engine to be harmonised with that of the car. The gears may be engaged or shifted by hand or operated by an automatic To-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
