Overdrive (mechanics)

Common card · No. 327068

Overdrive is a mechanical unit containing epicyclic gears sized to allow an automobile to cruise at a sustained speed with reduced engine speed (rpm), leading to improved fuel consumption and reduced wear and noise level. The term is ambiguous. The gear ratio between engine and wheels causes the vehicle to be over-geared, and cannot reach its potential top speed, i.e., the car could travel faster if it were in a lower gear, with the engine turning at higher RPM. The power produced by an engine increases with the engine's RPM to a maximum, then falls away. The point of maximum power is somewhat lower than the absolute maximum engine speed to which it is limited, the "redline". A car's speed is limited by the power required to drive it against air resistance, which increases with speed. At the maximum possible speed, the engine is running at its point of maximum power, or power peak, and the car is traveling at the speed where air resistance equals that maximum power. There is therefore one...

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