Overdrive is a feature in a car that allows the engine to operate at a lower RPM for a given road speed, which can lead to better fuel efficiency, lower noise, and lower wear). It is essentially a high gear mode for a vehicle and can be thought of as an added gear that widens the transmissions range. When overdrive is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached (usually 70+ km/h ).

Here are some key points about overdrive in a car:

• Overdrive is the highest gear in the transmission).
• It allows the transmission to go to the highest gear, which can be fourth or fifth gear, depending on the car.
• Overdrive is any gear that is numerically less than a 1:1 ratio.
• It is possible for a car to have multiple overdrive gears.
• Overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behind the transmission unit).
• An overdrive can either couple the input driveshaft directly to the output shaft (or propeller shaft) (1:1), or increase the output speed so that it turns faster than the input shaft (1:1 + n) ).
• Overdrive is not a thing anymore in modern cars, as the separate gearbox and final drive have merged into a single transaxle.

In summary, overdrive is a feature in a car that allows the engine to operate at a lower RPM for a given road speed, which can lead to better fuel efficiency, lower noise, and lower wear. It is essentially a high gear mode for a vehicle and can be thought of as an added gear that widens the transmissions range.