Spur gears are the most common type of gears. They have straight teeth, and are mounted on parallel shafts. Many spur gears may be used at once to create very large gear reductions.

Spur gears are used in many devices that you use each day: windup alarm clock, washing machine, electric screwdriver, oscillating sprinkler...
But you won't find many in your car...

This is because the spur gear can be noisy. Each time a gear tooth engages a tooth on the other gear, the teeth collide, and this impact makes a noise. It also increases the stress on the gear teeth.

To reduce the noise and stress in the gears, most of the gears in your car are helical gears.