Overdrive

Overdrive describes any gear whose ratio is numerically below 1:1, meaning the gearbox output shaft rotates faster than its input. In practical terms the engine turns more slowly than the propeller shaft or driven wheels, so a car cruising on the motorway can hold a high road speed while the crankshaft spins at relatively modest revolutions. The concept exists to break the historical link between road speed and engine speed: without an overdrive ratio the engine would have to scream along at high rpm on the open road, burning more fuel and wearing faster than necessary.

Mechanically, overdrive can be delivered in two ways. The original approach, common from the 1930s through to the 1980s, was a separate epicyclic unit bolted behind the main gearbox, engaged by a solenoid or electric switch and acting on the top one or two ratios. A sun-and-planet gear set, locked or freed by a hydraulic clutch and brake band, stepped the output speed up above input speed. The modern approach simply designs the top gear or gears of the main gearbox with a ratio below 1:1, so the function is built in rather than bolted on; a six-speed manual might have a fifth around 0.85:1 and a sixth nearer 0.65:1.

The benefits are quieter, more refined and more economical cruising. Dropping engine speed from, say, 3,500 rpm to 2,200 rpm at a steady 110 km/h reduces friction losses, pumping work and noise, which can cut motorway fuel consumption noticeably and ease long-distance fatigue. It also reduces engine wear over the life of the car, since fewer revolutions are accumulated for every kilometre travelled.

The trade-off is torque. Because the ratio multiplies speed at the expense of turning force, an overdrive gear leaves little reserve for acceleration or climbing. Asking the car to pull away up a gradient in overdrive will labour the engine, so the gear naturally drops out, by driver action in a manual or by automatic kickdown, whenever brisk acceleration or a steep hill demands more pulling power. On older separate-unit systems the overdrive was usually inhibited in the lower gears for exactly this reason.

Understanding overdrive clarifies several related ideas. It is one component of a car's overall gear ratio spread, working with the final-drive ratio of the differential to set engine speed for a given road speed. Modern eight, nine and ten-speed automatics effectively contain multiple overdrive ratios, allowing very long-legged cruising, while continuously variable transmissions achieve the same relaxed effect without discrete steps. In every case the underlying purpose remains the historical one: letting the engine loaf along economically once the car is up to speed.