In automatic transmissions, overdrive describes a gear ratio that is below 1-1 engine speed-to-transmission speed. This lower ratio offers superior fuel economy but poor acceleration and usually engages only when cruising speeds have been reached. Different automatic transmissions achieve this gear ratio in different ways -- some with an extra gear, others by restricting slippage in the transfer case. In either case, overdrive should be used unless hauling or driving in very mountainous terrain.
Many automatic transmissions, particularly in smaller sedans, have a three-speed arrangement with an optional fourth gear that is referred to as overdrive. This gear engages automatically unless overridden by the driver. In some cars, an electronic switch disengages overdrive. In others, drivers can shift into either drive or overdrive. Shifting through drive into overdrive permits the transmission to engage this fourth gear, and the transmission will shift through the first three gears as normal.
Semi-tractors and some heavy-duty pickup trucks and SUVs have a transfer case that transfers power from the transmission to a drive axle. Transfer cases have a second clutch that allows some slip between gears in the transmission and the drive axle. In this configuration, overdrive releases the clutch and locks these gears together. When overdrive is engaged in a transfer case, it allows higher mileage but can overheat a transmission if engaged under high loads.
Designed for hauling and towing, most late-model heavy-duty pickup trucks have five- or six-speed transmissions with an automatic overdrive. These transmissions are programmed to engage the overdrive gear when a cruising speed has been achieved and the engine can comfortably carry a load without overreaching. Overdrive in these configurations can usually be switched off if the truck is hauling a heavy load, particularly through hilly terrain. In general, overdrive should be disengaged when towing.
Overdrive gets its name from the sense that the engine is over-geared, meaning the vehicle could achieve a higher speed if it were in a lower gear. The final drive in most transmission designs has a 1-1 gear ratio, meaning one revolution of the transmission translates into one revolution of the drive axle. Overdrive gears drop that ratio to below 1-1. These lower ratios generally fall in the 0.7-1 range, which means that 0.7 of a revolution of the transmission result in one revolution of the drive axle.
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