The principle of operation of a single-stage gearbox of an automotive gearbox.
The drive from the engine via the clutch is transmitted to the gearbox via a shaft and a small gear, which has, for example, 15 teeth. This gear meshes with the larger gear, mounted on the shaft transmitting drive to the vehicle wheels, which has e.g.. 30 teeth. Therefore, two revolutions of the shaft, transmitting drive from the engine, the drive shaft will only make one revolution. In this case, the driving force on the shaft, on which the tire is permanently attached to the vehicle, is twice the driving force on the shaft.
In the example described, the gear ratio is 2 (30:15 = 2). Gear ratio, As you know, is the ratio of the number of teeth of both gears or the ratio of the rotational speed of the driven shaft to the rotational speed of the driving shaft.