Differential gear ratios

[MountainManJoe]

The easiest way to determine your final drive or gear ratio is to look at 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 on newer ones. (see photo)

You should find one of these RPO codes...

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

And one of these...
GQ1 - Open differential (standard)
G80 - Eaton Gov-Loc limited slip differential (optional)

There are other ways if your sticker is missing or unreadable:

1. You may have a different code stamped on you axle tube. If you can find it under the rust, it can be used to find your gear ratio.

2. You may have a stamp on your differential ring gear, and sometimes the pinion gear, in the form pp-rr

This differential has 11 pinion gear teeth and 37 ring gear teeth.

where pp=number of pinion gear teeth, rr=number of ring (or crown) gear teeth, and your ratio = rr÷pp

3. With the rear end in the air and transmission in neutral, turn both wheels at the same time and in the same direction, and watch the pinion shaft (this may not be possible with AWD). Count the number of times the wheels rotates to make the pinion make one full turn. This is your ratio.

[MountainManJoe]

Additional information that would be helpful to this topic:

- 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
- Did 1985-91 AWD vans come with 4.10 gearing from the factory, and are the ring & pinions compatible with later years?

[doyoulikeithere]

TB, The AWD option was never offered until at least 1990, so 85 to 89 is just no to AWD.
I I have tried before to upload a PDF as an attachment, because I have a pretty complete list of all the RPO codes here, but cant upload it to the thread. The extension pdf is not allowed.
Oh Well, Here's Photos of some RPO stickers.

1995 AWD

1987 5 Speed

[doyoulikeithere]

hey, looky what I found... Actually, I knew it was there for ten years now....

Heres the link, and a copy of the goods from that link... http://www.astrosafari.com/old/ib/ AstroZam, Justice, I'm sure y'all remember that place eh?
If you click on the tech info tab, you find rear axle info as well as some stuff about how to do a gen 1 to Gen 2 nose swap, dutch doors from barn doors etc. Its all good stuff.
Note: I would be shot for doing just this at "that other site".....

The RPO CODE on the sticker only shows which rear axle was installed at the factory. If you are not the original owner of your van you don't know for sure if the axle has been replaced or not. ONE way to be almost SURE what your rear axle ratio is will be to look at the actual factory code stamped on the axle itself. But the only certain way is to loo at the numbers stamped on the crown and pinion gears...

AXLE TUBE CODES: The surefire way to find out which gear ratio your rear axle has is to look at the spot you see pointed out in the image above. The parking brake cable runs right across the area you need to see. You will need to clean off the axle tube well to see the letters-numbers clearly. You will notice the code is located on the front of the tube on the passengers side of the van. However, I have found that on 1988 and older, the code is found on the front of the drivers side axle tube. NOTE! The first 3 letters of the code stamp are the ones that matter. Just compare them to the list provided below to determine you gear ratio.

FAA: 7.5", 2.56 open (1985-1986 Astro)
FAF: 7.5", 2.73 open (1988 Astro)
FAG: 7.5", 3.08 open (1988-1989 Astro)
FAH: 7.5", 3.42 open (1988-1989 Astro)
FAJ: 7.5", 2.73 posi (1988 Astro)
FAK: 7.5", 3.08 posi (1988-1989 Astro)
FAL: 7.5", 3.42 posi (1988-1989 Astro)
FAM: 7.5", 3.73 open (1988-1989 Astro)
FAN: 7.5", 3.73 posi (1988-1989 Astro)
FAP: 7.5", 4.10 open (1988-1989 Astro)
FAR: 7.5", 4.10 posi (1988-1989 Astro)
FAS: 7.5", 3.23 open (1988-1989 Astro)
FAT: 7.5", 3.23 posi (1988-1989 Astro)
FBA: 7.5", 2.56 posi (1985-1986 Astro)
FCA: 7.5", 2.73 open (1985-1987 Astro)
FCB: 7.5", 2.73 posi (1985-1986 Astro)
FFA: 7.5", 3.08 open (1985-1987 Astro)
FFB: 7.5", 3.08 posi (1985-1986 Astro)
FMA: 7.5", 3.42 open (1985-1987 Astro)
FMB: 7.5", 3.42 posi (1985-1986 Astro)
FRA: 7.5", 3.73 open (1985-1987 Astro)
FRB: 7.5", 3.73 posi (1985-1986 Astro)
FSA: 7.5", 4.11 open (1985-1986 Astro)
FSD: 7.5", 4.11 posi (1985-1986 Astro)

GWH: 7.5", 4.10 open (1985-1987 Astro)
GWJ: 7.5", 4.10 posi (1985-1986 Astro)

HZA: 7.5", 2.73 posi (1987 Astro)
HZB: 7.5", 3.08 posi (1987 Astro)
HZC: 7.5", 3.42 posi (1987 Astro)
HZF: 7.5", 3.73 posi (1986 Astro)
HZG: 7.5", 4.11 posi (1986-1987 Astro)

KJM: 7.5", 2.73 open (1987 Astro)
KJN: 7.5", 3.08 open (1987 Astro)
KJP: 7.5", 3.42 open (1987 Astro)
KJR: 7.5", 2.73 posi (1987 Astro)
KJS: 7.5", 3.08 posi (1987 Astro)
KJT: 7.5", 3.42 posi (1987 Astro)
KJU: 7.5", 3.73 open (1987 Astro)
KJW: 7.5", 3.73 posi (1987 Astro)
KJX: 7.5", 4.10 open (1987 Astro)
KJY: 7.5", 4.10 posi (1987 Astro)
KRK: 7.5", 2.73 open (1987 Astro)
KRL: 7.5", 3.08 open (1987 Astro)
KRM: 7.5", 3.42 open (1987 Astro)
KRN: 7.5", 2.73 posi (1987 Astro)
KRP: 7.5", 3.08 posi (1987 Astro)
KRR: 7.5", 3.42 posi (1987 Astro)
KRS: 7.5", 3.73 open (1987 Astro)
KRT: 7.5", 3.73 posi (1987 Astro)
KRU: 7.5", 4.10 open (1987 Astro)
KRW: 7.5", 4.10 posi (1987 Astro)

PZK: 7.625", 4.10 open (1990 Astro)
PZL: 7.625", 4.10 posi (1990 Astro)

RHB: 7.625", 3.23 open (1990 Astro)
RHC: 7.625", 3.23 posi (1990 Astro)
RHD: 7.625", 3.42 open (1990 Astro)
RHF: 7.625", 3.42 posi (1990 Astro)
RHG: 7.625", 3.73 open (1990 Astro)
RHH: 7.625", 3.73 posi (1990 Astro)
RHL: 7.625", 3.23 open (1990 Astro)
RHM: 7.625", 3.42 open (1990 Astro)
RHN: 7.625", 3.73 open (1990 Astro)
RHP: 7.625", 3.23 posi (1990 Astro)
RHR: 7.625", 3.42 posi (1990 Astro)
RHS: 7.625", 3.73 posi (1990 Astro)
RHT: 7.625", 4.10 open (1990 Astro)
RHU: 7.625", 4.10 posi (1990 Astro)

SGF: 7.625", 3.23 open (1991 Astro)
SGG: 7.625", 3.23 posi (1991 Astro)
SGH: 7.625", 3.42 open (1991 Astro)
SGJ: 7.625", 3.42 posi (1991 Astro)
SGK: 7.625", 3.73 open (1991 Astro)
SGL: 7.625", 3.73 posi (1991 Astro)
SGM: 7.625", 4.10 open (1991 Astro)
SGN: 7.625", 4.10 posi (1991 Astro)
SGS: 7.625", 3.23 open (1991 Astro)
SGT: 7.625", 3.23 posi (1991 Astro)
SGU: 7.625", 3.42 open (1991 Astro)
SGW: 7.625", 3.42 posi (1991 Astro)
SGX: 7.625", 3.73 open (1991 Astro)
SGY: 7.625", 3.73 posi (1991 Astro)
SHD: 7.625", 3.23 open (1990 Astro)
SHF: 7.625", 3.23 posi (1990 Astro)
SHG: 7.625", 3.42 open (1990 Astro)
SHH: 7.625", 3.42 posi (1990 Astro)
SHJ: 7.625", 3.73 open (1990 Astro)
SHK: 7.625", 3.73 posi (1990 Astro)
SHL: 7.625", 4.10 open (1990 Astro)
SHM: 7.625", 4.10 posi (1990 Astro)
SHR: 7.625", 3.23 open (1990 Astro)
SHS: 7.625", 3.23 posi (1990 Astro)
SHT: 7.625", 3.42 open (1990 Astro)
SHU: 7.625", 3.42 posi (1990 Astro)
SHW: 7.625", 3.73 open (1990 Astro)
SHX: 7.625", 3.73 posi (1990 Astro)
SHY: 7.625", 4.10 open (1990 Astro)
SHZ: 7.625", 4.10 posi (1990 Astro)

TZD: 7.625", 3.23 open (late 1995 Astro)
TZF: 7.625", 3.23 posi (late 1995 Astro)
TZG: 7.625", 3.42 open (early 1995 Astro)
TZH: 7.625", 3.42 posi (late 1995 Astro)
TZJ: 7.625", 3.73 open (late 1995 Astro)
TZK: 7.625", 3.73 posi (late 1995 Astro)

USN: 7.625", 3.23 posi (1993-early 1995 Astro)

UTM: 7.625", 3.23 posi (1991-1992 Astro)
UTN: 7.625", 3.42 posi (1993-early 1995 Astro)
UTP: 7.625", 3.42 posi (1991 Astro)
UTR: 7.625", 3.73 open ('91-early '95 Astro)
UTS: 7.625", 3.73 posi ('91-early '95 Astro)
UTT: 7.625", 3.23 open (1991-1992 Astro)
UTU: 7.625", 3.23 posi (1991-1992 Astro)
UTW: 7.625", 3.42 open (1991-1992 Astro)
UTX: 7.625", 3.42 posi (1991-1992 Astro)
UTY: 7.625", open (1991-1992 Astro)
UTZ: 7.625", 3.73 posi (1991-1992 Astro)

VAC: 7.625", 3.23 open (1996-1997 Astro)
VBC: 7.625", 3.23 posi (1996-1997 Astro)
VCC: 7.625", 3.42 open (1996-1997 Astro)
VDC: 7.625", 3.42 posi (1996-1997 Astro)
VFC: 7.625", 3.73 open (1996-1997 Astro)
VGC: 7.625", 3.73 posi (1996-1997 Astro)
VKA: 7.625", 3.42 posi (1995 Astro-Export)
VKC: 7.625", 3.42 posi (1996-1997 Astro-Export)
VLA: 7.625", 3.73 posi (1995 Astro-Export)
VLC: 7.625", 3.73 posi (1996-1997 Astro-Export)

[MountainManJoe]

The AWD option was never offered until at least 1990

duh I knew that. Blonde moment. So I guess the only possibility for a set of front+rear factory 4.10's would be out of a 1990/91 AWD van. That means they are probably hard to find, but I'm going to keep an eye out at the junkyard anyway. These vans used 7.6" ring gears from 1989-2005, so they probably interchange. I wonder what other vehicles you can salvage 4.10's out of.

Nice find with the axle codes, DYLIH, and thanks for those pictures too.

[doyoulikeithere]

I wonder what other vehicles you can salvage 4.10's out of. Nice find with the axle codes, DYLIH, and thanks for those pictures too.

I think you can buy brand new 410 gear sets reasonably cheep if you really want them. Didja clicky the linky? Theres a pile of V8 swap info on that same page too.
try these just be sure to get the right ones for your axle, either 7.5" or 7.65"
http://www.performance4trucks.com/Drive ... DGM7.5-410

[MountainManJoe]

I reclaimed some pictures for us

Location of axle tube code:

Here's another part diagram but from the angle you see when you take the cover plate off.

What an open differential looks like. Notice it only has side & spider gears in the carrier.

This one has the Gov-loc unit. You can see the governor and locking pawl in front of the gears, and you can sort of see the clutch pack at the end of the carrier.

And here's a photo of my axle housing opened up. You can just barely see the 11-41 stamp on the pinion gear. (click to enlarge)

[LiftedAWDAstro]

So I guess the only possibility for a set of front+rear factory 4.10's would be out of a 1990/91 AWD van. That means they are probably hard to find, but I'm going to keep an eye out at the junkyard anyway. These vans used 7.6" ring gears from 1989-2005, so they probably interchange. I wonder what other vehicles you can salvage 4.10's out of.

That is easy...you can use any ring and pinion out of an S-10 axle. It doesn't matter if it's front or rear. The front has always been a 7.25" reverse rotation and early versions interchange with newer diffs. You can also grab front gears from a Trailblazer or Envoy as they use the same 7.25" high pinion diff. The diff itself is not useable as these go through the engine block. These vehicles are offered in 3.42, 3.73 and 4.10 ratios from the factory. The rear axle of the Trailblazer and Envoy are either an 8.0" or 8.6" so they are not useable.

The rears being a 7.5" or a 7.625" does not matter. They are also interchangeable through the years. The only thing to watch on the rear diff is the carrier break. If you have 3.08's or 3.23's you have a series 2 carrier and you can not bolt 3.42 and lower gears onto it. Same goes if you have 3.42's, 3.73's and 4.10's you can't bolt a set of 3.23's onto your series 3 carrier.

[WE7X]

Thanks to all for the excellent pictures!
I had seen it mentioned that there was a code stamped on the axle housing, but darned if I could find it. The picture of its location was just what I needed.
I finally was able to see the codes stamped on the axle tube on all three of my rear diff units. It had been looking for it, but did not realize it was a 'dot matrix' print... a series of punch marks making the characters like an old dot-matrix printer.
I recently purchased a used Torsen unit off E-bay, and it is now here and looks great; so my spare 3.73 rear diff will be headed of the the shop for bearings, seals and installation. I have done similar units on foreign cars before, but I have decided my time is better utilized doing things I can do efficiently.

[Astrophysics]

Hi All,
Is the 4.10 ratio reliable, or should I get 3.73?
I have 2” lift and 245/75-16 cooper discovery tires.
2003 Astro with np231 manual shift.

AP

[MountainManJoe]

[Herbie]

The chart says it all, but I'll echo with my own endorsement. I'm running 245/70-16 (about a 1/2" shorter than yours) and I'm very happy at 4.10.

"Reliable" isn't really a problem until you get to the very short ratios or running a TON of power. Anything in the 3.23 to 4.11 (or even a bit shorter) range is stout enough for the normal life of the van, as long as properly lubricated.

[Astrophysics]

Hi All,
Great info on axle ratios.
Thank you,

AP

[Bret Schmerker]

...
FAA: 7.5", 2.56 open (1985-1986 Astro)
FAF: 7.5", 2.73 open (1988 Astro)
FAG: 7.5", 3.08 open (1988-1989 Astro)
FAH: 7.5", 3.42 open (1988-1989 Astro)
FAJ: 7.5", 2.73 posi (1988 Astro)
FAK: 7.5", 3.08 posi (1988-1989 Astro)
FAL: 7.5", 3.42 posi (1988-1989 Astro)
FAM: 7.5", 3.73 open (1988-1989 Astro)
FAN: 7.5", 3.73 posi (1988-1989 Astro)
FAP: 7.5", 4.10 open (1988-1989 Astro)
FAR: 7.5", 4.10 posi (1988-1989 Astro)
FAS: 7.5", 3.23 open (1988-1989 Astro)
FAT: 7.5", 3.23 posi (1988-1989 Astro)
FBA: 7.5", 2.56 posi (1985-1986 Astro)
FCA: 7.5", 2.73 open (1985-1987 Astro)
FCB: 7.5", 2.73 posi (1985-1986 Astro)
FFA: 7.5", 3.08 open (1985-1987 Astro)
FFB: 7.5", 3.08 posi (1985-1986 Astro)
FMA: 7.5", 3.42 open (1985-1987 Astro)
FMB: 7.5", 3.42 posi (1985-1986 Astro)
FRA: 7.5", 3.73 open (1985-1987 Astro)
FRB: 7.5", 3.73 posi (1985-1986 Astro)
FSA: 7.5", 4.11 open (1985-1986 Astro)
FSD: 7.5", 4.11 posi (1985-1986 Astro)

GWH: 7.5", 4.10 open (1985-1987 Astro)
GWJ: 7.5", 4.10 posi (1985-1986 Astro)

HZA: 7.5", 2.73 posi (1987 Astro)
HZB: 7.5", 3.08 posi (1987 Astro)
HZC: 7.5", 3.42 posi (1987 Astro)
HZF: 7.5", 3.73 posi (1986 Astro)
HZG: 7.5", 4.11 posi (1986-1987 Astro)

KJM: 7.5", 2.73 open (1987 Astro)
KJN: 7.5", 3.08 open (1987 Astro)
KJP: 7.5", 3.42 open (1987 Astro)
KJR: 7.5", 2.73 posi (1987 Astro)
KJS: 7.5", 3.08 posi (1987 Astro)
KJT: 7.5", 3.42 posi (1987 Astro)
KJU: 7.5", 3.73 open (1987 Astro)
KJW: 7.5", 3.73 posi (1987 Astro)
KJX: 7.5", 4.10 open (1987 Astro)
KJY: 7.5", 4.10 posi (1987 Astro)
KRK: 7.5", 2.73 open (1987 Astro)
KRL: 7.5", 3.08 open (1987 Astro)
KRM: 7.5", 3.42 open (1987 Astro)
KRN: 7.5", 2.73 posi (1987 Astro)
KRP: 7.5", 3.08 posi (1987 Astro)
KRR: 7.5", 3.42 posi (1987 Astro)
KRS: 7.5", 3.73 open (1987 Astro)
KRT: 7.5", 3.73 posi (1987 Astro)
KRU: 7.5", 4.10 open (1987 Astro)
KRW: 7.5", 4.10 posi (1987 Astro)

PZK: 7.625", 4.10 open (1990 Astro)
PZL: 7.625", 4.10 posi (1990 Astro)

RHB: 7.625", 3.23 open (1990 Astro)
RHC: 7.625", 3.23 posi (1990 Astro)
RHD: 7.625", 3.42 open (1990 Astro)
RHF: 7.625", 3.42 posi (1990 Astro)
RHG: 7.625", 3.73 open (1990 Astro)
RHH: 7.625", 3.73 posi (1990 Astro)
RHL: 7.625", 3.23 open (1990 Astro)
RHM: 7.625", 3.42 open (1990 Astro)
RHN: 7.625", 3.73 open (1990 Astro)
RHP: 7.625", 3.23 posi (1990 Astro)
RHR: 7.625", 3.42 posi (1990 Astro)
RHS: 7.625", 3.73 posi (1990 Astro)
RHT: 7.625", 4.10 open (1990 Astro)
RHU: 7.625", 4.10 posi (1990 Astro)

SGF: 7.625", 3.23 open (1991 Astro)
SGG: 7.625", 3.23 posi (1991 Astro)
SGH: 7.625", 3.42 open (1991 Astro)
SGJ: 7.625", 3.42 posi (1991 Astro)
SGK: 7.625", 3.73 open (1991 Astro)
SGL: 7.625", 3.73 posi (1991 Astro)
SGM: 7.625", 4.10 open (1991 Astro)
SGN: 7.625", 4.10 posi (1991 Astro)
SGS: 7.625", 3.23 open (1991 Astro)
SGT: 7.625", 3.23 posi (1991 Astro)
SGU: 7.625", 3.42 open (1991 Astro)
SGW: 7.625", 3.42 posi (1991 Astro)
SGX: 7.625", 3.73 open (1991 Astro)
SGY: 7.625", 3.73 posi (1991 Astro)
SHD: 7.625", 3.23 open (1990 Astro)
SHF: 7.625", 3.23 posi (1990 Astro)
SHG: 7.625", 3.42 open (1990 Astro)
SHH: 7.625", 3.42 posi (1990 Astro)
SHJ: 7.625", 3.73 open (1990 Astro)
SHK: 7.625", 3.73 posi (1990 Astro)
SHL: 7.625", 4.10 open (1990 Astro)
SHM: 7.625", 4.10 posi (1990 Astro)
SHR: 7.625", 3.23 open (1990 Astro)
SHS: 7.625", 3.23 posi (1990 Astro)
SHT: 7.625", 3.42 open (1990 Astro)
SHU: 7.625", 3.42 posi (1990 Astro)
SHW: 7.625", 3.73 open (1990 Astro)
SHX: 7.625", 3.73 posi (1990 Astro)
SHY: 7.625", 4.10 open (1990 Astro)
SHZ: 7.625", 4.10 posi (1990 Astro)

TZD: 7.625", 3.23 open (late 1995 Astro)
TZF: 7.625", 3.23 posi (late 1995 Astro)
TZG: 7.625", 3.42 open (early 1995 Astro)
TZH: 7.625", 3.42 posi (late 1995 Astro)
TZJ: 7.625", 3.73 open (late 1995 Astro)
TZK: 7.625", 3.73 posi (late 1995 Astro)

USN: 7.625", 3.23 posi (1993-early 1995 Astro)

UTM: 7.625", 3.23 posi (1991-1992 Astro)
UTN: 7.625", 3.42 posi (1993-early 1995 Astro)
UTP: 7.625", 3.42 posi (1991 Astro)
UTR: 7.625", 3.73 open ('91-early '95 Astro)
UTS: 7.625", 3.73 posi ('91-early '95 Astro)
UTT: 7.625", 3.23 open (1991-1992 Astro)
UTU: 7.625", 3.23 posi (1991-1992 Astro)
UTW: 7.625", 3.42 open (1991-1992 Astro)
UTX: 7.625", 3.42 posi (1991-1992 Astro)
UTY: 7.625", open (1991-1992 Astro)
UTZ: 7.625", 3.73 posi (1991-1992 Astro)

VAC: 7.625", 3.23 open (1996-1997 Astro)
VBC: 7.625", 3.23 posi (1996-1997 Astro)
VCC: 7.625", 3.42 open (1996-1997 Astro)
VDC: 7.625", 3.42 posi (1996-1997 Astro)
VFC: 7.625", 3.73 open (1996-1997 Astro)
VGC: 7.625", 3.73 posi (1996-1997 Astro)
VKA: 7.625", 3.42 posi (1995 Astro-Export)
VKC: 7.625", 3.42 posi (1996-1997 Astro-Export)
VLA: 7.625", 3.73 posi (1995 Astro-Export)
VLC: 7.625", 3.73 posi (1996-1997 Astro-Export)

Thanks for the reference chart. The stock 7.625" American Axle ten-bolt my M11006 should have "VFC" on the iron label the right tube. Essential information for how configured at Baltimore (MD, USA) Assembly. Currie Enterprises has, I estimate, two 3.73:1 ring-and-pinion sets (one high-pinion for the front axle) to support the 9" Improved dropout, which in my case will require unequal-length shafts and tubes for the starboard offset o' the engine and gearbox.