I recently had the rear differential replaced in my 02 Envoy and I believe they installed a differential with the wrong gear set. My envoy came with the tow package and 4:10 gears. When I test drove the Envoy I noticed that my speedometer was 10 MPH off. My GPS said I was going 70 and the speedo indicated 60. This is a 4 wheel drive unit and when I put it in 4 whd (in snow) it pulled hard to the right almost into a ditch. What I am asking is, are there marking on the rear housing to indicate what gear set is inside? I need evidence to take this back to the repair shop.

 

When you jack up the wheels, you can count it. Rotate your wheels one full turn and your driveshaft will make 4.1 turns assuming you have 4.1 gears.

You could also remove the the differential cover...