At travel speeds below this maximum, there is a range of gear ratios that can match engine power to air resistance, and the most fuel efficient is the one that results in the lowest engine speed. There is therefore one specific gear ratio at which the car can achieve its maximum speed: the one that matches that engine speed with that travel 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. A car's speed is limited by the power required to drive it against air resistance, which increases with speed. The point of maximum power is somewhat lower than the absolute maximum RPM to which the engine is limited, the " redline" RPM. The power produced by an engine increases with the engine's RPM to a maximum, then falls away. the car could travel faster if it were in a lower gear, with the engine turning at higher RPM. The most fundamental meaning is that of an overall gear ratio between engine and wheels, such that the car is over-geared, and cannot reach its potential top speed, i.e.
Overdrive is the operation of an automobile cruising at sustained speed with reduced engine revolutions per minute (RPM), leading to better fuel consumption, lower noise, and lower wear. 1960s Triumph gearbox with Laycock de Normanville electro-hydraulic operated overdrive Laycock de Normanville "J type" overdrive unit A plate warns to only engage the unit in third and fourth gears. Please help improve this article by introducing citations to additional sources.įind sources: "Overdrive" mechanics – news Īn overdrive is sometimes a separate unit that fits into the back of a gearbox, as with this Fairey unit.
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