General Motors Differential Axle Codes

What is my rear differential ratio?

The table below will help you determine what rear end ratio you have by using your RPO code, answering the question: What rear end ratio was in my Chevrolet or GMC vehicle when it left the factory. You can typically find your RPO code on driver side door panel or glove box.

General Motors rear end differentials are referred to by the diameter of the ring gear, i.e. 7.5 inch, 8.5 inch, 10.5 inch.  Our experts will help you find the correct ring and pinion set for your particular application.  Give us a call at 800-216-1632 to get started. We have a large selection of gear sets in stock and can recommend the correct gear ratio to match your engine horsepower, transmission specs and tire size to give you the desired performance.

Switching your rear end gear ratio helps your gas mileage or improves your power!

If you're looking for better gas mileage, you want to switch to a higher gear ratio.  If your looking for more power or performance, you would switch to a lower ratio.  It's just backwards from the way we think, if you have a 4.10 ratio and need better fuel mileage you would switch to a 3.42 or 3.73 gear ratio.  If you have a 3.42 rear end ratio and need more towing power you would switch to a 4.10 ratio .  If you have installed larger tires, they rob performance, and you would want to go to a lower ratio like a 4.56 or 4.88 depending upon your tire diameter.

Want to search for rear end parts by application, ask the Gear Wizzard

We specialize in matching your tire size and ring and pinion ratio for your specific application and needs. Thinking about changing tire sizes? We can help! Consult our Look up chart for determining 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.

Why you need a positraction?

Without positraction only one rear wheel receives the power.  In bad road conditions like mud, ice or snow, power to one wheel is bound to get you stuck.  Installing a positraction distributes power to both wheels at the same time, causing a significant improvement in traction.  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/gasket shapes, which is a common technique to identifing differential ring gear diameter.  And a Dana/Spicer Parts illustration showing how to locate the Dana bill of material number is provided below. This is a critical number to ensure that you find correct Dana parts the first time.  For example, in some cases, Dana used 3 to 4 different ring gear widths for a Dana 35 ring gear. So the set up is not the same for each gear category. 

When ordering Dana Spicer Differential Parts, provide a bill of material number. This number is stamped on the right hand side axle tube of your rear end. It's a number like: 603XX-X, 604XX-X or 605XX-X. With this number, you can tell exactly what components the factory used in the assembly of your rear end.