Axle Ratio

Armyfoo · September 1, 2016

I just discovered this evening that I have a rear axle issue. With the truck jacked up, grabbing the tire top/bottom and moving it, the entire axle shaft is "tilting" so to speak up and down. Granted, I had an experienced "mechanic" looking this over right along with me and now I know where my "growling/grinding" sound is coming from. Going in reverse, it is very grabby and clunky sounding.

To the point of the matter.

Any ideas what is wrong before I pop the cover off and begin looking around in there at something I have no idea what I am looking at?

The data tag is worn down on my door, so I have no idea what my ratio's are. How can I find out what the ratio is?

SteveBBB · September 1, 2016

To find you gear ratio, jack up the truck, mark your tire and drive shaft, turn tire one full turn and keep track of how many turns the drive shaft makes. Close to 3 1/2 turns would be 3.42, close to 3 3/4 turns would be 3.73 etc.

Mr. Z71 · September 1, 2016

To determine your axle ratio, you can find the appropriate three digit RPO code on your service parts identification label (found adhered in your glove box). Then you can decode it by an online search of GM RPO codes or using the RPO Code Lookup tool located under Tools on the Menu bar at the top of this page. This is assuming that the rear axle ratio has never been changed since the factory vehicle build.

Two examples are my trucks, one has a "GT4" RPO code which says it has a 3.73 axle ratio, and the other has a "GU6" RPO code meaning the truck has a 3.42 axle ratio. There are several codes for the many ratios. This is the easiest and cleanest way (no dirty work necessary) to determine your ratio, other than calling a dealer and giving them your VIN.

As to your problem, without inspecting it myself, I would guess that you have a bad bearing, either an axle shaft bearing, carrier bearing, or possibly both. Only a guess, though, based on my own previous experience. Your first signs will be metal flakes in the oil once you pull the cover.

Post what you find out, just curious.