This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. Overdrive allows the engine to operate at a lower rpm for a given road speed. Most automatic transmissions have three speeds and overdrive (fourth speed). Generally speaking, overdrive (OD) is the highest gear in the transmission. This trend has rendered the overdrive a complex and obsolete solution to economy gearing in automobiles, and very few cars are fitted with them today, although they still appear on large trucks, where more gear ratios are always in demand. However, automotive manual transmissions manufactured since the 1980s tend to include a larger selection of gear ratios than before, the highest of which is usually greater than 1:1. This left an unfulfilled need for a higher gear ratio for highway cruising, which was filled by the addition of separate overdrive units. Early manual automotive transmissions were limited to three or sometimes four speeds, with only the most sophisticated being five speeds.
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