平歯車

A gear is a toothed wheel that transmits rotation and torque by meshing with another gear.

Meshing gears of different sizes change the speed and torque between shafts, letting the drivetrain match engine output to the load and road speed; tooth profile and material are matched to the load carried.

Gears are found in the gearbox, differential, and various drives and pumps. The correct gear depends on the assembly, the tooth count, and the matching gears it runs with.