I noticed there's a lot of people posting stuff about posi's and gear ratios and etc... I also noticed, there's no stickies on this stuff, so why not try and get one? So if we get a nice collection of info we could probably get a sticky going.

First off, RPO codes and gear sizes.

GU2 - 2.73
GU4 - 3.08
GU5 - 3.23
GU6 - 3.42
GW6 - 3.27
GM3 - 3.45
GT4 - 3.73
G80 - posi
G92 - performance

Also check here if you want.

Gear teeth count (incase you don't have an RPO code like me)
Ring Pinion Gear Ratio
41 - 15... 2.73
40 - 13... 3.08
42 - 13... 3.23
41 - 12... 3.42
41 - 11... 3.73
41 - 10... 4.10

Posi units and Limited Slip Differentials:

Posi=Limited slip incase no one knows. Means both wheels lock together when driving straight and unlock around corners.

From 82-88 and 89 they had 26 spline differential carriers. Meaning the axles themselves had 26 splines that went into the carrier or differential. from 88 and 89 to 92, they had 28 spline axles. 2 series carriers will fit 3.08 and numerically lower gears. 3 series carriers fit 3.23 and numerically higher. 2 series carriers can still fit a 3 series gear but needs a spacer for the gear. Also the lower the gear the faster your top speed will be but slower the take off, higher the gear the faster your take off is, but slower top speed.

This is all I can think of at this time, I will probably add more later. Feel free to add any more information, tips, tricks or correct my errors. Transmission information could be added here too. Enjoy!(I tried to make this as idiot proof as possible)

 

The 2 series carrier will fit 3.08 and higher gears. The 3 series carrier will fit 3.23 and lower gears.

 

It's doubtfull you're going to find 28 spline axles in a 88 or 89 unless you have a 9 bolt. Possibly in a very late 89 10 bolt but don't assume it's 28 spline. In 1990 they became standard.

Posi is a GM term for a limited slip differential. Ford and Chrysler have their own terms for the same thing.

82 to about 86 has a 7.5" ring gear. 87 and up 10 bolts have a 7.625" ring gear. Both fit the same carrier.

 

My bad on that, it was late and I was tired. But i fixed it.