How to Find the Gear Ratio on a GM Rear End

by Don Bowman

There are a couple of ways to find the gear ratio on a GM rear end. If changing the gear ratio is desired, it is important to know exactly which rear housing is present. They come in different sizes, and the ring and pinion necessary to change the ratio are specific to these sizes. The RPO code located in the glove compartment or in the trunk on most GM vehicles is needed to identify the type of rear end. There are 164 different RPO codes, so the dealer, service facility or a reference book will be needed to interpret these codes.

Step 1

Raise and support the rear of the vehicle with jack stands. Read the metal tag attached to one of the differential cover bolts. This tag, which is always present (if not removed by a previous owner), will indicate the gear ratio that came with the vehicle originally. If the vehicle was purchased secondhand, this tag may not be valid if the previous owner has changed the ratio, and you will have to remove the cover to find the gear ratio.

Step 2

Place a drip pan under the differential cover. Remove the differential cover (it simply unbolts) so you can determine the exact gear ratio and whether the differential is a limited slip or an open differential.

Step 3

Count the teeth on the small gear in the forward part of the axle housing, called the pinion gear. Count the number of the teeth in the big ring gear attached to the differential. Divide the number of teeth on the pinion into the number of teeth on the ring gear. This is the gear ratio.

Step 4

Clean the gasket material from the differential cover (use the gasket scraper). Wipe the mounting surface of the cover with a cloth. Apply a 1/8-inch bead of RTV sealant around the circumference of the cover. Allow the sealant to skin-over for 10 minutes before installing the cover.

Step 5

Install the cover and the bolts. Tighten the bolts with the 3/8-inch socket and ratchet. Remove the filler plug on the side of the axle housing above and to the rear of the pinion. Fill the differential with the two bottles of differential oil.

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