What Gear Ratio is My Rearend?

You can find your gear ratio easily with this method. All you need is a jack, some jackstands, and knowing how to count helps too.

You're going to have to put the car in nuetral or out of gear, so safety first! Do this on level ground, and be sure and chock the front wheels so the car can't move. It helps to have someone there in the car just in case...

First, do you have a posi or limited slip rear? If you're not sure, just jack up the rear so both tires are off the ground and put your jackstands under the axle. Now turn one of the tires. If the other tire turns the same way or doesn't turn at all, you should have a posi or limited slip. If the tire turns opposite of the way the first tire spins, then it's probably an open rear end.

If you have a car with a posi or limited slip rear, jack up the car and get both rear tires off the ground. Now put your jackstands under the axle. Make a line on the pinion yoke and onto the rear end with chalk or a grease pencil. Do the same to the tire on the car, so you can see when the tire has rotated exactly one turn.

Now count the number of turns of the driveshaft that it takes (use your chalk marks) to make the rear tire turn one complete rotation. (Transmission needs to be in nuetral or out of gear.)

If your car has an open rear, jack up only one rear tire and leave the other on the ground. Now put a jackstand under the lifted side. Turn the tire and again count the number of turns of the driveshaft, but this time you must turn the tire two full rotations.

So:

Posi: Jack both wheels off the ground. Turn one wheel 1 complete turn.

- Count the turns on the drive shaft.

Non-posi: Jack one wheel off the ground. Turn the wheel 2 complete turns.

- Count the turns on the drive shaft.

If it takes about 2 and three-quarters turns, it's around a 2.73 gear ratio. If it's a little more than 3, you have 3.08s. If it takes about 3 and a quarter turns of the driveshaft, you have 3.23 or 3.25 gears. Very close to 3 and three-quarter turns, 3.70 or 3.73 gears. This method won't work very well on a posi that's worn-out and won't spin both tires equally.

This method is very handy when looking through junkyards or swap meets. It's good to be able to figure what gear ratio you have without tearing anything apart.

This is a quick and easy way to tell what gear ratio is in your rearend. You might not be able to tell exactly what gear you have, but you'll be very close.

If you want to be accurate, then count the teeth.

If you have the rearend opened up where you can see the gears, count the teeth on the ring and pinion. Divide the ring teeth number by the pinion teeth number.

Example: 41 teeth on the ring and 10 teeth on the pinion = 4.10 gear ratio...


Return from Gear Ratio to Drivetrain

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