How to Determine Rear End Ratio

by Thomas West

The rear end (or rear axle) of rear-driven vehicles may be available with different axle ratios. If the vehicle will be towing a trailer, then a lower ratio may be desired. If economy is your aim, you may want a higher ratio. If you do not know the axle ratio of your vehicle, the most accurate way to determine it is to divide the number of teeth on the ring gear by the number of teeth on the pinion gear. These parts are internal to the rear end and are not exposed unless the rear end is disassembled. However, by following a few steps, the rear end ratio can be determined within a reasonable degree of accuracy without disassembling the axle.

Chock the front wheels of the vehicle. Raise the rear of the vehicle with a hydraulic floor jack and place heavy-duty jack stands under each side of the rear axle housing. Lower the jack and remove it.

Place a chalk mark on the vehicle's drive shaft underneath the vehicle so that when the drive shaft turns, you can count the number of revolutions the drive shaft makes.

Turn the driver's-side rear tire a quarter turn in a counterclockwise direction to remove the slack from the drive train.

Make a chalk mark on the driver's-side rear tire in the six o'clock (bottom) position.

Use the chalk mark on the tire as a reference and turn the driver's-side rear tire in a counterclockwise direction one complete revolution. Watch the chalk mark on the drive shaft and note how many revolutions it makes while you turn the tire. Use a helper if you cannot see the drive shaft while turning the tire. Be as accurate as possible.

Determine the rear axle ratio from the number of drive shaft revolutions (e.g., three-and-a-half drive shaft revolutions would indicate a 3.50 axle ratio, four-and-a-quarter revolutions would indicate a 4.25 axle ratio, etc.).

Raise the rear of the vehicle and remove the jack stands. Lower the rear of the vehicle to the ground. Remove the wheel chocks from the front wheels.

Tip

• check This process will work whether you have an automatic or a manual transmission.