Help with axle number

[Milo]

I have searched high and low and I can not, for the life of me figure out what gear ration this stamping indicates.

Best I can get is:

AN 080W82

This is in a 1978 GMC Jimmy 4wd, 350, Auto

The RPO list in the glovebox was not helpful.

Also, it has an RPO of MX1. If I follow that means there is a TH350 and a NP #203. Does that sound correct?

 

[PrairieDrifter]

To the best of my knowledge, the numbers in the castings can only tell you the year and maybe some manufacturing code? But other than that it won't tell you anything about the axle or gear ratio.

Rpo codes or get to counting teeth!

 

[SquareRoot]

Anything will be a guess. Pull the cover and verify. It probably needs a fluid change anyway.

 

[Bextreme04]

There used to be a thread that had all of the axle codes, but it seems to have disappeared... @HotRodPC any ideas where it went? I found a thread that linked to it, but when you click the link it shows that it doesn't exist anymore

 

[Keith Seymore]

I have searched high and low and I can not, for the life of me figure out what gear ration this stamping indicates.

Best I can get is:

AN 080W82

This is in a 1978 GMC Jimmy 4wd, 350, Auto

The RPO list in the glovebox was not helpful.

Also, it has an RPO of MX1. If I follow that means there is a TH350 and a NP #203. Does that sound correct?

Here's a few from another site. I don't think they include squares yet:

Rear Axle Ratio Charts For 1967 - 1972 GM Trucks - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Rear Axle Ratio Charts For 1967 - 1972 GM Trucks The 1967 - 1972 Chevrolet & GMC Pickups Message Board

67-72chevytrucks.com

Rear Axle Codes - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Rear Axle Codes The 1967 - 1972 Chevrolet & GMC Pickups Message Board

67-72chevytrucks.com

Help ID this rear end - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Help ID this rear end The 1973 - 1987 Chevrolet & GMC Squarebody Pickups Message Board

67-72chevytrucks.com

MX1 is the "Merchandised" RPO for a non-overdrive automatic (similarly - MX0 would be for an overdrive auto).

The engineering release charts would then flow down to the "non-merchandised" specific trans RPO based on model, series, engine, GVW, etc.

I don't recall what the non-merchandised RPO is for a TH350: M38? I recall M40 is the TH400.

Another example:

Let's say you want a manual trans, three on the tree. You tell your dealer that's what you want and he chooses "MM3". Based on that, and the rest of your order, it flows down to a M62 or M64 non merchandised option, depending on first gear ratio that was spec'd by engineering for your combination.

K

 

[WFO]

I have searched high and low and I can not, for the life of me figure out what gear ration this stamping indicates.

Best I can get is:

AN 080W82

This is in a 1978 GMC Jimmy 4wd, 350, Auto

The RPO list in the glovebox was not helpful.

Also, it has an RPO of MX1. If I follow that means there is a TH350 and a NP #203. Does that sound correct?

I just jack up the rear and spin the rear wheel 1 complete revolution while counting the revolutions of the driveshaft if it's a positrac. Around 3 3/4 revolutions would be a 3.73, and a little over 4 would be a 4.10.

If it's not a positrac leave one wheel on the ground, or have someone hold it still while counting driveshaft revolutions.
Or you can just let the other wheel spin the opposite direction and double the driveshaft revolution number.

 

[Craig 85]

I can tell you I had a '77 K5 and a '79 K15, both were 350/350/NP203 California trucks. Both of them had 3.08 gears. Both got axle swapped to 3/4 tons with 4.10's.