How to Calculate the Rear Axle Gear Ratioby Thomas West
The rear axle gear ratio is the number of turns that the input of the differential at the driveshaft makes in relation to its output at the rear wheels. Different gear ratios can have different effects in the way a vehicle performs. If you have ever wondered what the gear ratio is in your rear-wheel drive vehicle and the factory axle tag is missing, there is a simple method of calculating this without disassembling the rear axle.
Slide a floor jack under the rear of the vehicle and raise it until the rear wheels are off the ground. Place jack stands under the rear axle on both sides of the vehicle. Lower the jack so the weight of the rear of the vehicle is supported by the jack stands.
Crawl under the vehicle and locate the drive shaft where it attaches to the front of the rear axle. Place a 6 to 8 inch length of masking tape longitudinally on the drive shaft where it will be easily visible from the side of the vehicle near one of the rear wheels.
Place a piece of masking tape on the edge of one of the rear tires.
Rotate the tire by hand one complete revolution using the piece of masking tape on the tire as a guide.
Count the number of revolutions the drive shaft turns (using the masking tape attached to it as a guide) to one tire revolution. For instance, if the drive shaft rotates three and a quarter turns to one tire rotation, then your rear axle gear ratio is 3.25:1.
Remove the masking tape from the drive shaft and tire. Raise the rear of the vehicle with the floor jack and remove the jack stands. Lower the floor jack until the rear wheels are on the ground.
- You can pinpoint the exact rear axle gear ratio of your vehicle by finding out what axle ratios were available for your particular vehicle from the owner's manual or a repair manual. Using the example in the above procedure, if ratios 2.80:1., 3.28:1 and 4:11.1 were available for your vehicle, you can determine that the 3.28:1 ratio is the one closest to your calculations.