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Determining rear end ratio

From: Dave L.
Email: davel at
Remote Name:
Date: July 25, 2003


I hope these answers I saved long ago help: Answer from Dick found on the Imperial web pages: "To check your rear axle ratio, jack up one side of the car (use a jack stand of course) and slither under far enough to put a chalk mark on the drive shaft. With the car in neutral (the front wheels blocked, of course) and the parking brake off, rotate the wheel that is off the ground two full turns, while watching the chalk mark on the drive shaft. Count how many turns the drive shaft turns in two complete wheel rotations, that will be your rear axle ratio within a few percent. If you have posi-traction, you'll feel some clunking and hear some noise when you do this, not to worry, that's normal." Or from the Portland AMX Club pages: "If you don't have the axle assembly apart, you can still do it. Block the front wheels and one of the rear wheels, then raise the other rear wheel off the ground so you can rotate it. (You blocked all the wheels because you are going to take it out of gear or out of park and release the parking brake.) Put the raised corner of the car on a jack stand or other secure stand. Don't trust the jack. Make a chalk mark on the drive shaft or rear universal joint and a matching mark on the differential housing. Make another chalk mark on the rear tire that's up in the air, lined up with something convenient on the car body. Now turn the wheel exactly two revolutions while counting the revolutions of the drive shaft. If the drive shaft made 3 1/2 turns while the wheel turned two revolutions, the ratio is approximately 3.50 to 1. You turned the rear wheel two revolutions, rather than one, because the differential action make the wheel turn twice as far as it would have if both wheels were turning simultaneously. If you have a limited-slip rear axle ("Positraction," Equa-Lock," "Sure-Grip," etc), you won't be able to turn one wheel at a time; they'll want to turn together. In this case, jack up both rear wheels and count the driveshaft revolutions while you turn the wheels one revolution."


Last changed: July 19, 2018