Overdrive is a gear in a cars transmission that allows the engine to operate at a lower RPM for a given road speed, which can lead to better fuel efficiency and quieter operation on the highway). Here are some key points about overdrive in a car:

• Overdrive is the highest gear in the transmission, with a ratio higher than the ratio for maximum speed).
• It can be looked at as an added gear that widens the transmissions functionality.
• When it is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached (usually 70+ km/h ).
• Overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behind the transmission unit).
• An overdrive allows the output shaft to be "overdriven" relative to the input shaft, meaning that the output speed may be faster than the input speed).
• Overdrive can be turned on and off in older cars, but in modern cars, it is usually always on.
• Overdrive is not a thing anymore in manual transmissions, but it is still present in some automatic transmissions.

In summary, overdrive is a gear in a cars transmission that allows the engine to operate at a lower RPM for a given road speed, leading to better fuel efficiency and quieter operation on the highway.