An overdrive is a device which was commonly used in automobiles to allow the choice of an extra-high overall gear ratio for high speed cruising, thus saving fuel, at the cost of less torque. Usually the final or top gear is called overdrive. Non direct drive gears increase torque multiplication at the expense of higher engine rpm and thus, decreased fuel economy.
Overdrive allows the engine to operate at a lower rpm for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. When it is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached (usually 70+ km/h [43+ mph] depending on the load). When it is off, the automatic transmission shifting is limited to the lower gears. For an automatic transmission, it is almost always best to select overdrive and allow the transmission to control engagement of the overdrive. (It may be necessary to switch it off if the vehicle is being operated in a mountainous area or when towing a trailer).
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Overdrive (mechanics) - Wikipedia, the free encyclopedia
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