How To: Determine the Rear Axle Ratio

 

Rear Axle Ratio

The rear axle ratio of your car is instrumental in determining which transmission gear set is best suited for your car. It is also important to know the ratio as well as tire size to determine which speedometer gear you need so your speedometer reads accurately.  By following these directions, you can determine what the axle ratio is in your car.


Determining a Rear Axle Ratio of a Posi Rear

  1. Safety first! Chock the front tires so your car won't slide off the jack stands then raise the rear of the car with a hydraulic floor jack and place heavy duty jack stands on either side of the rear axle housing.  Lower the car onto the jack stands.
  2. Turn the driver's side tire a quarter turn counterclockwise to remove the slack in the drivetrain.
  3. With a piece of chalk or masking (painters) tape make about a 6" line along the driveshaft that can be easily seen from the side of the car and make another mark on the tire.  It's good to make this mark in a place that is easy to line up, like the edge of the wheel well or the 12 o'clock position.
  4. Rotate the tire counterclockwise while counting the number of times the driveshaft rotates until the tire has completed one complete turn. (You may need a helper if you can't rotate the tire and see the driveshaft at the same time.) Be as accurate as possible.
  5. The number of driveshaft rotations will help you determine your rear axle ratio.  For example, if it turned 3 and a half times, it's a 3.50:1 ratio.  If it turned 4 and a quarter times, it's a 4.25:1 ratio.
  6. Remove the tape/marks from the driveshaft and tire, raise the rear tires again and remove the jack stands before lowering your car back to the ground.  Remove the chocks from the front tires.

 

Determining a Rear Axle Ratio of a Non-Posi Rear

  1. Jack one wheel off the ground, put the transmission in neutral and take the parking break off.  ***Be sure to use jack stands under the axle for safety precaution***
  2. Mark the wheel and the drive shaft with chalk or masking tape.
  3. Turn the wheel TWO revolutions and count the number of driveshaft turns.
  4. The number of driveshaft rotations will help you determine your rear axle ratio.  For example, if it turned 3 and a half times, it's a 3.50:1 ratio.  If it turned 4 and a quarter times, it's a 4.25:1 ratio.
  5. For better accuracy, turn the wheel 20 times and divide by 2 for your rear axle ratio.

 

If you are unsure whether you have a Posi Rear or not

Jack both wheels off the ground and turn one wheel.

  • If the opposite wheel turns the same direction or doesn't turn at all, then the rear axle is posi or limited slip.
  • If the opposite wheel turns the opposite direction, then it's probably an open rear axle (non-posi).