Kowing your truck’s axle ratio is important information if you are reprogramming you speedometer computer or want to replace the gears to retain your truck’s stock acceleration after installing larger or smaller tires. This is an important consideration, given the fact that increasing tire size effectively reduces your truck’s axle ratio, causing it to feel as if it had higher gearing than it does. While this can indeed be good for gas mileage, it does significantly decrease the amount of torque your truck puts to the ground, and thus its acceleration.
Chock the front tires, and ensure that the wheel chocks are securely bearing the weight of the vehicle.
Place the jack on one side of the axle, raise your vehicle so that the tire clears the ground by 2 to 3 inches and insert the jack stand.
Reverse operation on the other side of the vehicle, making sure the entire rear end is raised from the ground.
Duct tape the string end of the plumb bob to the fender above the tire, so that so the string lines up with the center of the wheel. Mark the tire beneath the string with the grease pencil so the two line up.
Clean the drive shaft with solvent, and mark it and the differential housing with a grease pencil to create alignment marks.
Measure the drive-shaft circumference in cm and note your measurement.
Have one assistant hold the unmarked wheel (if you’re using an open differential), while the other rotates the marked wheel counterclockwise until the reference mark comes back around to rest beneath string.
Count the number of times the drive-shaft reference mark passes the reference mark on the differential housing. The drive-shaft mark will pass the differential mark on the last revolution and come to rest past it.
Use the strap tape to measure the distance between the differential reference mark and drive-shaft mark in the direction of rotation.
Divide this measurement by the circumference of the drive-shaft, and add the whole revolution number, as in this example:
Drive-shaft circumference: 10 cm
Whole revolutions: 3
Distance past diff reference mark: 4 cm
10/4 = .25
3 + .25 = 3.25
Axle ratio = 3.25 to 1
Things You’ll Need
- Floor jack
- 2 Jack stands
- Strap-type measuring tape (metric)
- White grease pencil
- Duct tape
- Plumb bob
- 2 assistants
- Clean the drive-shaft with solvent if necessary so that the grease pencil will show. It might not be a bad idea to write the axle’s ratio on top of the differential housing for future reference. The grease pencil will weather well, and the marks should be visible years later.
- Always use jack stands and chock grounded wheels when working under any vehicle. Never trust the parking brake or parking gear in the transmission to prevent rolling.