Ring and pinion gears are used to spin wheels and axles forwards and backwards, with their most common usage in motor vehicles to improve drive-train performance and provide extra power to the wheels.
A ring and pinion gear set comes with a grooved ring connected to the drive axles of the vehicle's rear wheels and a grooved pinion connected to the drive shaft. Any vehicle with rear-wheel drive, or even older cars, come equipped with ring and pinion gears. However, these gears can be replaced with sets with a higher ratio in order to improve performance.
The ring and pinion work together to create torque when the driver puts power to the drive wheels. As the drive shaft, and thus the pinion, spin, the ring, and thus the drive axle and the wheels, spin correspondingly. The larger the ratio of the ring and pinion grooves, the more force is created with each spin and more power goes to the wheels.
Ring and pinion gears must have proper teeth ratios or they will not work properly over time. A ring of one size and a pinion of another size might work for awhile, but continued use will cause failure because the grooves will not fit together and they will grind until they are ruined. The pinion always has fewer teeth/grooves than the ring, and a desirable ratio is anywhere between 3.4 to 3.6.