Information about Differentials in GM vehicles can usually be obtained by looking at the RPO or “Regular Production Option” codes contained in the Service Parts Identification tag in the glovebox. Whenever possible, reference the tag numbers when ordering parts for your Differential.

For a front thru-axle, this is measured from the inside to inside of your fork. For a rear thru-axle, this is measured from the inside to inside of your frame at the drop-outs. The O.L.D. measurement is listed for many thru-axles, but isn’t necessary if you know the overall length.

How to Measure Rear End Width

1. Locate the axle flanges on either side of the rear axle. The flange is the platform on which the wheels are mounted and bolted to the axle.
2. Using a measuring tape, measure from one flange to the other.

Measure the ring gear diameter. The ring gear is the largest gear inside the differential housing and normally faces the driver’s-side rear wheel. Using a tape measure or machinist’s ruler, measure the ring gear diameter. This identifies the differential — a Ford 9-inch or GM 7.5-inch — for example.

Sure a 9 inch can handle 400+ horse in a car.

S10 2WD’s have a drum to drum mounting width of 54.5″ which is perfect for a T-Bucket with larger back wheels. The Blazer and 4WD models have about 4.5-5″ more overall width and works well for 26 and newer fender-less roadsters, and 23 style T’s with narrower rear wheels.

Rear axle widths. These dimensions do not include brake drums and are measured from backing plate to backing plate. Actual wheel track could be as much as 6″ to 8″depending on drum size: 46″ to 47″. ’66 to ’77 Bronco. 48″ to 49″. ’72 to ’75 Datsun pickup.

These dimensions do not include brake drums and are measured from backing plate to backing plate. Actual wheel track could be as much as 6″ to 8″depending on drum size:

Using the charts below, you will be able to learn alot about the rear differential in your classic full size Chevy. The gear ratio codes are located on the front passenger side of the differential as shown in the illustration below. This code will tell you the ratio, whether it is posi or not, any special notes and what plant it was made at.

60″ 60’s GMC and some Chevy’s Dana 44 or Dana 60 with leaf springs 6 lug, Gear ratio’s 3.21 to 3.92. GMC V-6 used a 3.21 Dana 44 used a standard chevy1310 yoke, Dana 60 used the heavy duty yoke which takes the larger 1350 yoke.