Thread: GM differencials

Tweet

On the rear axle tube on the passenger side on the side facing forward will be a three letter code.

GT4-3.73
GT5-4.10
GT7-3.33
GT8-4.10

GU1-2.41
GU2-2.73
GU4-3.08
GU5-3.23
GU6-3.42
G80-Positraction

HO4-4.11

If there is no code on the tube (like mine), chalk mark the rear tire, have a couple of your friends push the car forward SLOWLY, ONE COMPLETE REVOLUTION, as you lay next to it and count the revolutions of the driveshaft. Do it a few times if you have to. It will give you a close approximation of your ratio. Don't try to jack up one wheel and turn it, it won't give you the ratio.
Look at the brake backing plate:
If you have the 3 bolt triangular pattern it's a 26 spline axle, 7.50.
If you have the 4 bolt square pattern it's a 28 spline axle, 7.625.
Anything newer than 88 is usually a 28 spline/7.625.
On the 4-bangers with a stick trans, the rears were usually 3.73.
On the 4.3 V6 with the TH700 4R, the rears were either 3.08 or 3.42.
The 4.11 ratio will give you a neck-snapping take-off, but poor highway speeds and poor MPG. You'll be revving at about 3300 RPM to do 60 MPH. The 3.73 isn't much better. The 3.42 will rev at about 2600 at 65 MPH, and the 3.08 will do better. With a 3.42 or 3.08 you'll get a moderate take-off and good highway speeds with better fuel economy.
If you need to go with a wider rear, the 4X4 rears are 4" wider.

That's all the info I can remember right now.