Do you need to know your differential ring and pinion gear ratio?
Often there is a tag on the differential which will tell you your gear ratio. If the tag is missing or you believe the gears have been changed, then you can determine your gear ratio several different ways.
If you know your speedometer is correct, you can use a chart. If you know the gears have been changed, but no modifications have been done to recalibrate the speedometer readings, then there is a very good chance that your speedometer is no longer accurate.
If your speedometer is not correct, you can determine the gear ratio by comparing the rotations of the driveshaft, or pinion gear, and the wheels, or ring gear. You will need to lift the rear tires off the ground and rotate the driveshaft. Count the rotations of the driveshaft and the wheels. Divide the number of rotations of the tires by the rotations of the driveshaft. This will give you a ring and pinion gear ratio.
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