Gear ratio check
Methods to determine the gear ratio of the differential
Counting the teeth on ring and pinion
If there is a removable cover on the differential, removing it and counting the teeth on the gears is the most reliable method. Count the teeth on the ring gear and the pinion gear, then divide the ring gear tooth number by the pinion tooth number to find the ratio.
- For example:
41 divided by 10 = a 4.10 ratio
40 divided by 13 = 3.08
On some ring gears, there will be a pair of numbers stamped into the outer circumference that will be the tooth count. It may look like 41:10 or 41-10, etc.
Ring gear stamped with tooth count: 45-11 = 4.09:1 ratio, for a Dana 44 differential.
Rotating the tires method
With a standard open (non posi traction) differential raise one wheel off of the ground, mark the tire at the 6:00 position, mark the drive shaft. Turn the tire exactly 2 full turns. Count the number of times that the drive shaft turns.
2 3/4 turns of the drive shaft = 2.73, just past 3 turns = 3.08, 3 1/3 = 3.23 or 3.31, almost 3 1/2 = 3.42, just past 3 1/2 = 3.55, 3 3/4 = 3.73, just past 4 = 4.10 or 4.11.
With a good solid limited slip differential (posi) raise both wheels off of the ground and turn both wheels exactly 1 full turn. Everything else is the same as above. You can also do this with a standard differential as long as both wheels turn exactly 1 full turn.
This is much easier with 2 or 3 people.
- First method
1. Drive the vehicle at a steady speed. Note the RPM.
- For example, if the vehicle is driven at 50 mph, and at that speed the tachometer reads 2,000 RPM, record that the engine speed was 2,000 RPM at 50 mph.
2. Measure a drive tire’s diameter in inches.
3. Multiply the RPM by the diameter of the tire.
- For example, if the diameter of the tire was 30 inches, 30 multiplied by 2,000 equals 60,000.
4. Multiply the speed at which the vehicle was driven by 336. In this example, the vehicle was driven at 50 mph. So, 50 is multiplied by 336 to equal 16,800.
Divide the figure produced in Step 3 by the number produced in Step 4 to determine the rear end gear ratio. In this example, 60,000 is divided by 16,800 to equal 3.57. Therefore, the rear end gear ratio is 3.57:1. 60,000 / 16,800= 3.57 or 3.57:1 gear ratio.
- Second method
Another easy way to estimate the final drive ratio is to use a gear ratio calculator.
Drive the vehicle at 50 MPH (as an example), note and enter the speed (50 mph), the RPM (what ever you noted), and for the final drive, enter 1 into the calculator. When you hit calculate, the rear gear ratio estimate will come up.