How to Decode a Mustang Rear Endby Lee Sallings
The Mustang came from the factory with a 7.5-inch or 8.8-inch rear end, after 1979, with a variety of gear ratios and in both limited slip and open differential configurations. Decoding the data tag on the rear end will provide the information required when ordering replacement or up-graded parts for the Ford differential inside the rear end. The average Mustang enthusiast can decode the rear end data tag in just a few minutes.
Park the Mustang on level ground and wedge wheel chocks in front of the front tires. Lift the rear of the Mustang using a floor jack under the differential housing. Place jack stands under the Mustang's rear axle tubes and lower the car onto the stands for support.
Slide under the car and clean off any accumulation of grease and dirt with a shop towel. Remove the 13 mm bolt, using a 13 mm wrench, that attaches the data tag to the differential, if the first part of the code on the tag is hidden by the bolt head.
Decode the data tag. The first line of the data is the axle code and may be needed when ordering axles and other rear end parts from the factory. The second line of data indicates the gear ratio, ring gear size, and differential configuration (open or limited slip). For example: A second line of data that reads "3L08 88 ***" would be a gear ratio of 3.08-to-1, a ring gear size of 8.8-inches, and the "L" in the first four digits indicates a limited slip differential. If the space where the "L" is in this example is blank it is an open differential and if the "88" is replaced with a "75" it has a 7.5-inch ring gear size.