How do I find my rear differential ratio?

How does switching your rear end ration help the driver or vehicle performance?

Thinking about changing tire size we can help, consult our Look up chart for determination of engine RPM, gear ratio and tire size combination. Larger tires on lifted truck suffer from loss of power, changing the ring gear and pinion to a lower ratio is the answer.  Changing rear end ratio can improve fuel mileage, reduce and extend engine life, and/or add additional power for your towing requirements.

Why you need a positraction?

A positraction unit, when your vehicle is negotiating a corner, the outside wheel has to travel a grater distance than the inside wheel. Therefore, the outside wheel must turn faster than the inside wheel. The differential is the device within the axle assembly which, in addition to transmitting the power to each axle shaft/wheel, allows one wheel to turn at a different speed than the other.  Without positraction only one rear wheel receives the power.  In bad road conditions say, mud, ice or snow traction to one wheel is bound to get you stuck.  Installing a positraction provide power to both wheels at the same time, a significant improvement in traction.

General Motors Production Options. The following Regular Production Option (RPO) codes identify the rear axle and transaxle overall final drive gear Ratios. These codes can be located on the RPO sticker in the glove box or in the trunk on most GM vehicles.  Axle codes typically begin with the letter  F, G and H.  RPO Codes G80,, G81m and G86 indicated that the differential left the factory with a positraction.  For positraction replacements and parts see the following link.  Positraction units are available for many General Motor FWD Front Wheel Drive models.

A chart is also provided which presents Differential cover/cover gasket shapes, a common technique to identify differential ring gear diameter.  And a Dana/Spicer Parts illustration showing how to locate the Dana bill of material number is provided. This is a critical number to ensure that you find correct parts the first time.  For example Dana used in some cases 3 to 4 different ring gear widths for say a Dana 35 ring gear, so the set up is not the same for each gear category.

Find General Motors Differential Parts and Accessories at the following links:

• GM 8.875" 12 Bolt Truck Rear End Parts & Accessories