Overdrive (mechanics)

Overdrive (mechanics)

An overdrive (OD) is a mechanism that allows an automobile to cruise at sustained speed with reduced engine RPM, leading to better fuel economy, lower noise, and lower wear.

Use of the term is confused, as it is applied to several different, but related, meanings. The most fundamental meaning is that of an overall gear ratio between engine and wheels, such that the car is now over-geared and can no longer reach its potential top speed, i.e. the car could travel faster if it were in a lower gear, with the engine turning more quickly. In some cases this implies a second definition is also true, that the gearbox output driveshaft is rotating faster than the original engine RPM. This later definition was common in the past, but many cars no longer have a driveshaft and it is not technically correct.

In the era of front-engine, rear-wheel drive layouts, the device for achieving an overdrive transmission was usually a small separate gearbox, attached to the rear of the main gearbox and controlled by its own shift lever or electrical actuation button. These were often an optional extra on some models of the same car. As popular cars became faster relative to legal limits and fuel costs became more important, particularly after the 1973 oil crisis, the use of 5-speed gearboxes became more common in mass-market cars, with the 5th gear being an overdrive, eliminating the need for a separate gearbox.

With the popularity of front wheel drive cars, the separate gearbox and final drive have merged into a single transaxle. However the fundamental meaning, that of an overall ratio higher than the ratio for maximum speed, still applies. Although the deliberate labelling of an overdrive is now rare, the underlying feature is now found across all cars.