GM rear ends are stamped with identification on the axle housing. If you are standing at the rear of your truck, the ID number should be on the top side of the axle on the right side of the differential cover. Take the ID number and look in a GM Vintage ID Number Manual for the gear ratio of the rear end.
If the manual is not available, pull the differential cover off the rear axle. Count the teeth on the pinion gear and the ring gear. Divide the number of teeth on the ring gear by the number of teeth on the pinion gear. This will give you the gear ratio of your rear end. For example, 41 teeth on ring gear and 10 teeth on pinion gear would be 41 divided by 10 equals 4.10.
This is the most reliable method because the ring and pinion set up could have been changed, therefore the ID number stamped on the axle housing may not match the original set up.
Best of luck.
