DoctorDiff Axle Calculator

Step 1:
Choose Rearend


Tip: This calculator only works for cars built for "right-hand traffic."

Tip #2: If you have a Dana or any custom width rearend and want to run original Mopar adjustable tapered ("Set 7") wheel bearings, please call.

Step 2:
Choose Pinion Offset


Tip: Select "Measure existing housing or axles" to skip the pinion offset and enter the dimensions of your housing directly.

Tip #2: A pinion offset of "0" places the pinion in the exact center of the vehicle. A positive pinion offset moves the pinion toward the passenger side. A negative pinion offset moves the pinion toward the driver side.

Step 3:
Choose Measurements

Change Brake Gap?

Tip: For "A", "B", "C","D" and "brake gap" measurements, use the schematics below as references.

Tip #2: The Dana is measured from the BACK, so the passenger tube is on the right of drawing. 8 3/4" and 9" housings are measured from the FRONT (brake line clip on the back toward top), so the passenger tube is on the left.

Tip #3: DoctorDiff axles have 2.525" (Dana and 8 3/4") and 2.5" (Ford 9") brake gaps. If your brakes require a different gap, check the box above and enter a custom value.

Step 4: Enter Measurements
(if any)

Passenger Side (A)
Driver Side (B)
Housing Width (C)
Passenger axle
Driver axle
Pinion offset
Brake Gap Axle Flange to Axle Flange (D)
Nearest 1/16"

Step 5: Click "Calculate"


  1. If you use an existing housing, position it rightside up (brakeline clip toward top on 8 3/4" and Ford 9" housings) and verify that the A, B, and C dimensions above match what you have. If they do not match, you have either entered something incorrectly, or your combination will not work with your existing housing.
  2. Verify your brakes will work with the chosen brake gap. If the gap is too narrow, your brakes will not fit.
  3. To calculate "Drum to Drum" width, add 2 times the thickness of your brake drum to (D) dimension.