Help me decipher my parts truck (rear gear ratio - Dana 44)

[60short]

Hey All,

I bought a 1978 GMC C15 High Sierra, that somebody had started pulling apart as a way to help get my 1960 back on the road.

We have gotten down to the basics - I have the engine and subframe, as well as the rear end, but its current stored somewhere I can't get at it, and I am trying to do the math on gear ratios to help me finalize transmission point

All I have with me at the moment is a slightly fuzzy picture of the sticker with codes ....

The truck had leafs and what I believe was a Dana 44

With these option codes, can anybody help me sort the rear gear ratio from here?

A28
DG4
F44
L59
N33
T44
XLB
YF5
ZLB
ZY4
64W
A01
C60
B63
K30
MX1
N41
U35
XLB
YLB
ZQ2
Z62
68L

Thanks!

[roboticjunky]

None of those rpo codes are for the axle ratio. For the axle it would be Gxx or Hxx. Here's a listing of axle rpo codes.

https://www.differentials.com/gm-axl...ication-codes/

[kwmech]

A 1978 is not going to have a dana 44 rear axle. Most likely a 10 or 12 bolt. Post a pic of the rear diff cover. Pull the cover anyways to get numbers off the ring gear, do an inspection and change the fluid

[Keith Seymore]

Originally Posted by roboticjunky

None of those rpo codes are for the axle ratio. For the axle it would be Gxx or Hxx. Here's a listing of axle rpo codes.

https://www.differentials.com/gm-axl...ication-codes/

This is correct.

Additionally, if the truck has the base ratio it will not appear on the SPID or will show "GQ8 - Base ratio". If the person who ordered the truck had chosen an optional ratio then those typically do appear on the SPID with the corresponding RPO.

Per the powertrain chart in the generic info packet the base ratio for a '78 C10 with L59 and federal emissions is a 3.07 (HE3).

https://www.gm.com/content/dam/compa...olet-Truck.pdf

K

[Keith Seymore]

Originally Posted by kwmech

A 1978 is not going to have a dana 44 rear axle. Most likely a 10 or 12 bolt. Post a pic of the rear diff cover. Pull the cover anyways to get numbers off the ring gear, do an inspection and change the fluid

This is also true.

Probably your best bet since there are no guarantees the original ring and pinion are still in place after 45 years.

K

[60short]

does the fact that is a GMC (C15) vs a chevy (C10) make any difference

I am hoping to be where the truck is tomorrow, so I will take some better pictures, but here is what I have at the moment







Thanks for all of the responses so far!

[Keith Seymore]

Originally Posted by 60short

does the fact that is a GMC (C15) vs a chevy (C10) make any difference

Nope.

C10 vs C15 = marketing magic

K

[60short]

Originally Posted by Keith Seymore

Nope.

C10 vs C15 = marketing magic

K

Kind of what I figured, but I could't find anything concrete on the option lists for GMC vs Chevy - I will try and grab some pictures, or crack the diff open and count the ratio anyways - appreciate your help!

- another Keith

[Keith Seymore]

Originally Posted by 60short

Kind of what I figured, but I could't find anything concrete on the option lists for GMC vs Chevy

They are the same.

A GMC is a Chevy with option Z88 "Nameplate conversion" added.

(or - depending on your perspective - a Chevy is a GMC with option X88 added).



All the rest of the options add and subtract in the same way regardless of brand.

K

[60short]

ok, so I am seeing 14:43 = 3.07s open diff, is it just a "12 bolt" ?

[kwmech]

ok, so I am seeing 14:43 = 3.07s open diff, is it just a "12 bolt" ?

Yes it is, nothing wrong with it. They are a good axle

[60short]

Originally Posted by kwmech

Yes it is, nothing wrong with it. They are a good axle

Cool, thank you for confirming - it just helps me make sure I am ordering the right parts (trying to minimize time spent on the lift (re)ordering parts)