How to Measure Your Rear Gear Ratio

Steps to Determine Your Rear Gear Ratio

1. First there are three components you will see, the rear differential, the pinion gear, and the U-Joint assembly. You will have to mark a line connecting from the pinion gear to the differential housing. This mark is made to identify where the center is, or the count/start point so we can count the revolutions of the pinion gear. To mark it you can use either a bright crayon paint marker or even a piece of tape. 

2. Next make a mark on the the actual tire/rim and either the frame right next to it,  the brake caliper, or the drum. Just like the pinion gear mark, this mark is used so you can count the revolutions of the tire. 

3. Finally, you want to look at this pinion gear almost like a clock. As the tire is rotated you will count how many times the pinion gear rotates to determine your gear ratio. If your vehicle is not posi-traction you need to hold the other tire still so it will spin and rotate the actual pinion gear. 

If you look at your pinion gear and the mark that is on it has rotated three quarters all the way around then that would be a rear-end gear ratio of a 373. So in conclusion to calibrate and determine your differential rear gear ratio, rotate the tire two times fully around and count how many times it goes past the mark you created on the pinion gear and differential housing. Keep in mind it will not end up dead center in most cases but it will determine what your gear ratio is.