Why petrol cars need a gearbox and most electric cars barely do
A typical petrol engine spins somewhere between about 600 and 7,000 revolutions per minute, yet a car's wheels only need to turn from standstill up to around 1,800. An engine also delivers its best power in a narrow band. The gearbox bridges that mismatch, and electric motors mostly make it unnecessary.
At heart a transmission is just a set of gears working together to change speed, direction of rotation or torque. The idea is ancient: windmills, horse-driven machines and steam engines used right-angle drives and other gearing to run pumps, mills and hoists. For cars, Louis Renault is credited with inventing the gearbox, though Carl Benz had anticipated it in 1886, rigging sprockets, chains and a clutch to give his Patent-Motorwagen a second, lower gear.
Early drivers shifted by hand, sliding gears along shafts with a lever, and until the 1950s most cars lacked synchromesh, the mechanism that matches shaft speeds so a change goes in smoothly. Automatics appeared on the American market from 1939, using the engine's own power to pick ratios. Today more than two-thirds of cars worldwide are automatic, and nearly every new car sold in the United States. Most combustion cars now have between four and ten forward gears.
Racing keeps older ideas alive. Sequential gearboxes and so-called dog boxes skip synchromesh in favour of dog clutches, allowing very quick and even clutchless shifts. Their straight-cut gears are what give race cars that distinctive whine, rising in pitch as the wheels speed up. Many dog boxes put first gear off to the lower left, a dog-leg layout that lines up second and third for the most frequent racing shift.
Electric vehicles sidestep much of this. Their motors work efficiently across a wide range of speeds, right down to a standstill, and can reverse simply by changing direction electrically, so they usually use a single fixed ratio or two speeds and no reverse gear at all. Continuously variable designs, typically two pulleys joined by a belt or chain, blur the steps in yet another way.
Source: Transmission (mechanical device)
