How to Determine Rear Differential Size

by Floyd Drake III

The size of a rear differential, or rear end, is determined by the diameter of the ring gear inside the differential housing. To measure the ring gear diameter, the inside of the differential unit must be accessed, which can be done by opening either the rear inspection cover or by removing the axle and cover on the forward face of the housing, depending on the differential type. The two differential types are the removable-carrier, also called a front loader, and the integral-carrier type, or rear loader. Rear loaders have a bolted inspection cover on the rear of the housing, while front loaders have a dome with no bolts.

Step 1

Remove the rear inspection cover on the integral-carrier type differential. Using a socket wrench, remove the inspection cover by loosening the bolts holding the cover to the rear of the differential housing.

Step 2

Access the ring gear on the removable-carrier type differential. First, the driveshaft on the front of the differential housing needs to be removed by loosening the bolts holding it to the differential. When this is accomplished, remove the bolts holding the front cover to the differential housing. When this cover is removed, the ring gear can be accessed. On certain front loaders, some of the cover bolts cannot be removed with a socket wrench and require a box wrench for removal.

Step 3

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. A diagram of the ring gear is found in Resources.

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