OD stands for overdrive. It shows the functioning of your vehicle at a maintained speed with a low number of RPM (revolutions per minute).However, ‘overdrive’ has several meanings. Some people also regard overdrive as a different unit placed at the back of a vehicle’s gearbox. Amongst all these meanings, you can define OD off in a car as a unique gear in your vehicle whose aim is to reduce stress on your engine. This implies that the gear allows your engine to function optimally. Of course, your engine will produce more power when using low gear.