Determine the axle gear ratio on the drive axle of any vehicle by lifting the vehicle, placing it in neutral, spinning a tire by hand, and measuring the number of rotations the drive shaft spins during one tire rotation. This provides the axle gear ratio for the vehicle.
Lift the drive axle and related end of the vehicle off the ground using an automotive jack. This is the front axle on front-wheel drive vehicles, and the rear axle on rear-wheel drive vehicles. On four-wheel drive vehicles, all four wheels need to spin freely, often requiring a heavy-duty automotive lift. Place tape on the drive shaft in a position that is clearly visible from a tire on the axle, and mark a spot on the tire midway down one side. Spin the tire by hand, and count the number of revolutions the drive shaft makes during one tire revolution. Continue this through multiple revolutions to ensure accuracy. The number of revolutions, including partial revolutions, made by the shaft equals the axle gear ratio.
Alternatively, car owners who have gear assemblies in hand may count the number of teeth on the ring and piston gear set and use an online calculator, such as those at Differentials.com or SierraGear.com, to determine the gear ratio. These calculators divide the ring gear tooth count by the pinion tooth count to provide the gear ratio, and they work for multiple gear systems but require an accurate count of the number of teeth on each part.