1978 Trans Am 10 bolt posi questions

[Burd]

Look on the tube. Code is stamped, my 400 has a 2:56. Gotta go by the code on the tube.

 

[67johnny]

A factory rear disk brake package would have been a great score but ...
Put a yellow paint marker dab on the axle flange and pinion flange and give her a count.
Not 100% accurate but a good indication for sure!

 

[Eddie Akers]

I found this on Craigs list.
https://detroit.craigslist.org/okl/pts/d/farmington-gm-rear-end-differential-10/7056085648.html

I spoke to owner. He still has it. Says its a 10 bolt posi from a 1978 Trans Am but he wasn't sure of the gear ratio. He said he thought it was a 2:73. He said it was the OE rear end. He says it is in excellent condition and it does look pretty clean in the pics.

I did some searching and I dont see where the Trans Am came with a 2:73 posi. From the info I found it looks like it "could" be a 3:23 or 3:42 with a Pontiac "Safe T Trac" type limited slip?

Any one here know anything about what rear ends came stock in the 78 Trans Am? There are casting numbers included in the pics. But its all greek to me.

I'm considering this to put in my 78 Type cause it would save me some money. Just not sure if its a good investment. I was really looking for a true trac type posi and going with lower ratio like 3:91.
I'm going to be running this behind a 4l80E trans and stock LS3.

Hey this is Eddie I am wanting to get into these conversations with as many members as possible but I am having trouble understanding how to do things on this Z28 site thanks for any help

 

[badazz81z28]

The ratio is stamped on the ring gear, just pull the cover. It’s 100% accurate

A factory rear disk brake package would have been a great score but ...
Put a yellow paint marker dab on the axle flange and pinion flange and give her a count.
Not 100% accurate but a good indication for sure!

 

[Kevin McKeown]

I have a 1978 Trans Am that I bought new from Pellie's Pontiac in Highland Falls, NY in June 1978. I ordered the turbo 350 and asked for the Pontiac 400 engine. I was told that by then only 4 speed manuals got the Pontiac 400, automatics got the Oldsmobile 403. I insisted I wanted the Pontiac engine, and the dealer got it for me (they sold hundreds of Trans Ams each year, so probably had a lot of pull with Pontiac). It came with the 2.56 rear (posi), which worked out fine for me since I took it to Germany and cruised the autobahns with it. When I got back to the states, I decided I wanted to go to the 3.42 rear, and got it done. I was concerned about the math for converting my speedometer to make it accurate. Found out that if I removed a little in-line module to the transmission, my speedometer was dead nuts on. Apparently, the folks at Pontiac had the car originally designed for the 3.42 in the first place. Lucky me! BTW, still have the car!

 

[Fbird]

simple 2 series posi carrier diff with useless gears $200 all day long...

 

[badazz81z28]

simple 2 series posi carrier diff with useless gears $200 all day long...

A 2 series with crappy gears? It sits and doesn't sell.....IMHO.

 

[big gear head]

If it has 2.73 gears then it's not a 2 series, it's a 3 series.

 

[Fbird]

thought 2 series was 2.56 and 2.73.......then 3 series stared at 3.23

 

[big gear head]

Not with the 8.5. The 8.5 uses a 2 series for 2.56 and 2.41 and 3 series for 2.73 to 5.13.

The 7.5 uses a 2 series for 3.08 to 2.41 and a 3 series for 3.23 to 4.56. The 8.2 10 bolt uses a 2 series for 2.73 and a 3 series for 3.08 to 4.56. The 12 bolt uses a 2 series for 2.73 and 2.56, a 3 series for 3.07 to 3.73 and a 4 series for 4.10 to 6.xx.