Find your differential gear ratio

[LiftedAWDAstro]

To determine your gear ratio, check your RPO codes. This is a set of 3-character codes used by the manufacturer to indicate your vehicle configuration. For example paint colour, power options, etc. It is normally located in the glove box on older vans, and on the passenger door jamb on newer ones.

You will find one of these:

Code - Gear Ratio
---------------
GU4 - 3.08:1
GU5 - 3.23:1
GU6 - 3.42:1
GT4 - 3.73:1
GT5 - 4.10:1

And if you find G80, the van was equipped with the optional Eaton Gov-Loc limited slip differential.

You may have a different code stamped on you axle tube as well. If you can find it under the rust, it can be used to find your gear ratio as well. The only other way to find your ratio would be turning the wheel/axle by hand and counting rotations, or opening up the axle and counting teeth.

[doyoulikeithere]

All True...All good! there is also
GU4 3.08:1 (BTW, This was in my 87 5 speed)
GT5 4.10:1

You can also turn the wheels and count how far the pinion turns to get a rough idea.
3.5 turns would be a 3.42:1 but 3.75 turns would be a 3.73:1 and so on...

That RPO code sticker is great for finding out a lot of things about your van.
I downloaded a list.pdf of GM RPO codes , I like to have it.

[MountainManJoe]

Oops I meant to say turn the shaft, not axle. I didn't know Astrofaris came with 3.08 gears, but I'm pretty sure 4.1 wasn't an option.

[LiftedAWDAstro]

4.10's were optional in the 85 - 90/91 vans and was standard in the RS/GT vans. In my opinion 4.10's should have been standard in these heavy AWD vans.

PS. Made this a sticky...can't believe we hadn't done this earlier! :-k

[MountainManJoe]

4.10's were optional in the 85 - 90/91 vans and was standard in the RS/GT vans.

Did the AWD come with 4.10s?


Should we update the first post with the two additional codes?

[doyoulikeithere]

Just a FYI, my 1990 Safari GT has a 3.42:1 axle ratio, according to the RPO code in the glovebox.
Also, the RPO code for an Open diff (as opposed to a G80 Limited Slip) is GQ1, If memory serves me right.

[astroturf]

4.10's were optional in the 85 - 90/91 vans and was standard in the RS/GT vans.

Did the AWD come with 4.10s?


Should we update the first post with the two additional codes?

I think it would be a good idea.

[Cobra]

4.10's were optional in the 85 - 90/91 vans and was standard in the RS/GT vans. In my opinion 4.10's should have been standard in these heavy AWD vans.

PS. Made this a sticky...can't believe we hadn't done this earlier! :-k

It was done a very long time ago what i forgot to do was put a link to the info viewtopic.php?f=4&t=34
Counting teeth is a reliable way to find a gear ratio just count the number of teeth on the ring gear(R) and pinion(P) and divide to get the final gear ratio(F) R÷P=F

also this applies to the front axle as well so i took the liberty of moving it slightly

[doyoulikeithere]

Counting teeth is a reliable way to find a gear ratio just count the number of teeth on the ring gear(R) and pinion(P) and divide to get the final gear ratio(F) R÷P=F. also this applies to the front axle as well so i took the liberty of moving it slightly

Yes, and the ring gear should also have numbers stamped right on it (visible if turned to the right spot) which will spell out the number of teeth for the ring, and the corresponding pinion gear like this xx:xx fill in the x's with the number of teeth crown:pinion.

[MountainManJoe]

I made a mistake about the sticker being in the door jamb too. I'd like to revise the original post based on all the new information. (see following post)

Things that would also be helpful to add are:
- RPO sticker: photo of glove box location. year that it moved (does it correspond to genI/II
- Which ratios were standard, and which were optional
- Breakdown of the axle tube codes

and something I want to know personally:
Could you get 4.10 gears in a 1985-91 AWD van from the factory, and are they compatible with my 2000?