[78MayanW87]

Experts - I have attached a pic from the passenger side, forward facing axle tube rear end code stamp from my 1978 Trans Am. I am a little, no... a lot...confused. In trying to decode this, the coding convention doesn't seem to line up with the various references that I have looked up. If I just focus on the "PP" code, that would indicated that, for a '78, it is a 3.23 rear end. However, when I rotate the driveshaft and count the wheel revolutions, it measures out to roughly a 2.41. That could be off, but it is definitely not 3.23. Also, the "G" would seem to indicate (I think) that it is a GM axle housing rather than Pontiac. Maybe this is normal, but I would have expected a Pontiac code. I know that I can pull the rear cover and count ring and pinion teeth but before I did that I thought that I would reach out to the experts and see what you all thought... Fire away!

[scott70]

Is it posi?

[78MayanW87]

Originally Posted by scott70

Is it posi?

Yes, it is a posi. I should have clarified that.

[johnta1]

Type PP in this:

Axle Code Search

[78MayanW87]

4PP G342 2

[johnta1]

Click on the 'Axle Code Search' in my 1st post. It is a link.
Should have a input area. Type 4pp instead of the pp.

I think it is a 1979 3.23 axle.

Is your rear end a safety-track? (posi)

[78MayanW87]

Originally Posted by johnta1

Click on the 'Axle Code Search' in my 1st post. It is a link.
Should have a input area. Type 4pp instead of the pp.

I think it is a 1979 3.23 axle.

Is your rear end a safety-track? (posi)

It is a posi, but like I mentioned in my initial post, there are only about 2-1/2” driveshaft rotations per wheel rotation. What am I missing?

[PunchT37]

Isn’t ‘78 an oddball year with 3.42’s instead of 77 or 79’s 3.23’s?

Are all the autos 2.41’s ?

[phil400]

Originally Posted by PunchT37

Isn’t ‘78 an oddball year with 3.42’s instead of 77 or 79’s 3.23’s?

Are all the autos 2.41’s ?

Don't now about all autos, but in 78 the 4speed w72s got 3.42 and auto got 3.23, for 79 both got 3.23 like in 77.

[The Champ]

It's very possible that someone changed the original 3.23 gears to whatever gear ratio is in the car prior to you purchasing the car. Assuming your rough calculations are correct, maybe a previous owner wanted a higher top speed or better fuel mileage/lower reving engine.

[johnta1]

I'd agree with that.
Probably have to pop the cover off and count the teeth for actual ratio.
(and could probably tell if it is a GM gear)

[phil400]

How does the car feel? I mean a car with 3.23 or 3.42 will have alot more gitty-up from a dead stop over 2.41, what rpm you running at 70? That's another easy general way to tell..if you're spinning at or around 3k @70 it's definitely not a 2.41, imho

[mgarblik]

Honestly, why guess? 10 minutes and a $4.00 gasket and you have your answer for sure. Plus you can look at the teeth for condition, determine if the gear set is GM or aftermarket, you know for sure what you have. It's a 44 year old housing. May have had multiple gear changes by now. Let us know what you find.

[Skip Fix]

My 78 180 HP TA TH350 had 2.56s.

[78MayanW87]

Originally Posted by mgarblik

Honestly, why guess? 10 minutes and a $4.00 gasket and you have your answer for sure. Plus you can look at the teeth for condition, determine if the gear set is GM or aftermarket, you know for sure what you have. It's a 44 year old housing. May have had multiple gear changes by now. Let us know what you find.

Agreed. I initially was not able to see the axle code and so had just rotated the driveshaft and counted revolutions and came up with a rough 2.5 and so assumed that I had a 2.41 and was planning on changing that out to a lower gear. Yesterday I spent a little time cleaning up the axle housing and was able to read the code and was thrown off when I decoded as a 3.23 but non-Pontiac...hence the post. I will pull the cover off, count teeth and report back what I find. My guess is that someone, rather than changing out the ring and pinion as was suggested, replaced the entire rear end to achieve better fuel mileage. Although that runs counter to what I have found in the engine where they certainly weren't going after better fuel mileage. Who knows who, what or when any of this was done over the past 45 years! More to come and thanks to all for the input! Lee

[78MayanW87]

Ok guys - I pulled the cover and it looks like it is a 2.56 rear end. The code on the pinion gear is: 3984823-16:41-GM (see pic). As a reminder, the axle housing code was: 4PP G342 2. So, since I cant seem to match up a "4PP" code with a 2.56 rear end, it would seem that the gears were just swapped out of the original housing. I'm asking..... Also, any insight into the origin of the ring and pinion and what it may have come out of? Looks like it is a super common gear set but I didn't know if the code would give away any answers. Thanks for the help!

[Skip Fix]

I'll look in my 78 shop manual tonight it had a page with codes.

[johnta1]

What is the date code on the cowl tag?

[mgarblik]

If you take off the bearing caps and look next to the case bearings, you can tell if the gear set has been swapped or is OE to the housing. If there is a single cast iron shim on each side, it is OE. They spread the case and insert one shim per side OE. If there are multiple shims on each side, the case/gear set have been serviced/swapped at some time in it's life.

[Skip Fix]

All of the 78 codes start with a "2" for all body series. For posis 2PS -2.41, 2PT-2.56, 2PW 3.08, 2PX 3.23, 2 PY 3.42
Third letter identifies manufacturer G=Chevrolet Then day built of the model year(342), then shift-2