A device that may be used on rear-wheel drive vehicles to allow the choice of an extra-high overall gear ratio for high speed cruising, thus saving fuel, at the cost of less torque. It is engaged manually when under way.
Although this device can often be found on large trucks, where more gear ratios are always in demand, very few modern cars are fitted with separate overdrive units. Instead, the overdrive speed is typically achieved as a result of epicyclic gearsets that are integrated in the transmission. In these cases, there is no separately identifiable overdrive unit.
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