Why do cars have gears?

Cars work best when the motor is turning at medium speed - not too fast or too slow. Changing gears allows the motor to keep turning in the same speed range, while the car goes from very low speeds to highway speeds.

When two gears mesh together (so that one gear turns the other), they have what's called a "gear ratio", which is the number of teeth on one gear divided by the number of teeth on the other. When the gear that is connected to the motor (the "driving" gear) has few teeth, and the gear that is connected to the wheels (the "driven" gear) has many teeth, then the wheels turn slowly but with a lot of power. That's for going uphill or going faster. When the driving gear has many teeth and the driven gear has few, then the wheels turn much faster, but now it's much harder to climb a hill or to speed up.

A car transmission (also called a "gearbox") has a number of gears that can be put together in different combinations, to create different gear ratiOS; or even to change the direction of rotation so the wheels can go in reverse, although the motor keeps turning the same way.