Why do cars need gears?
Your engine spins at up to 7,000 turns a minute, but your wheels never need more than about 1,800. Gears bridge the gap.
▶ Start the storyCars need gears because an engine and a set of wheels want to turn at very different speeds. A petrol or diesel engine typically runs somewhere between about 600 and 7,000 turns a minute, but the wheels only need to turn between 0 and about 1,800. A transmission, or gearbox, is a set of two or more gears working together to change speed, direction of rotation, or the twisting force called .
600–7000 vs 0–1800
There is a second reason. An engine's and power change with how fast it spins, so there is a sweet spot, called its , where it works best. Several gear ratios let the car stay near that spot, which gives good power, good fuel use and a smooth ride, while still providing enough acceleration from a standstill and enough speed on a motorway.
You already know the idea from a bicycle. Low gears help you pull away or climb a hill, and high gears let you go fast once you are rolling. A car's gearbox does the same job for the engine.
The man who got there first was Carl Benz. In 1886 his first practical car used chains and a to give a second, low gear. Louis Renault, the founder of Renault, is credited with inventing the gearbox itself. Today most petrol and diesel passenger cars have between four and ten forward gears and one reverse gear.
Quiz me
0/3
Recap
Low gears give push at low speed; high gears give speed while keeping the engine in its power band.
💡 A trick to remember it · A gearbox is a translator between a chatty fast engine and slow, strong wheels.
Surprising fact · Electric cars usually have just one or two ratios and no reverse gear.
Sources (1)
No source, no claim. Every fact in this lesson (14 claims) cites at least one of these.