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An axle ratio or a gear ratio refers to the number of times the drive shaft rotates for every single rotation of the vehicle's drive tires. This axle ratio number is important in understanding a vehicle's performance potential as well as fuel consumption and engine speed at any given miles or kilometers per hour measurement. The typical, non-overdrive transmission final gearing output is one to one, or a 1:1 ratio of the engine and transmission output. Thus, with the engine and the transmission each revolving one full turn, the axle ratio determines how many revolutions the engine will need to make in order to rotate the drive tires one full turn. The more revolutions an engine makes to turn the wheels one full turn will result in a slower top speed and a greater amount of power sent to the tires.
In vehicles that will be required to pull heavier than normal loads, a higher axle ratio is commonly offered as an option from the vehicle manufacturer. Trucks and utility vehicles are often the recipient of these optional axle ratio packages, allowing a customer the choice of a factory stock working axle package or a specifically harder-working gear package that will enable the vehicle to pull harder at all speeds. In the heydays of the muscle car wars, the top vehicles in every manufacturer's lineup included optional axle ratio choices to allow for better drag racing speeds.
By adding a higher axle ratio to a car, the vehicle is able to accelerate off the starting line at a greater velocity, thereby allowing the vehicle to reach its top speed faster than a similarly-equipped vehicle with a lower gear ratio. This acceleration match-up is slower to react and come up to speed, and the lower axle ratio will provide a higher top speed. This is why the winning vehicle in a drag race may indeed have the slower top speed.
The ring and pinion gears in the drive axle determine the gear ratio of the drive axle, referring to the number of turns the pinion gear, or small gear, is required to make in order to turn the ring gear, or larger gear, one full turn. When attempting to find the axle ratio of an unknown gear set-up, it is possible to count the teeth on both the ring and pinion gears and divide the number of teeth on the ring gear by the number of teeth on the pinion gear. The result is the final gear ratio of the axle.