1978 Trans Am 10 bolt posi questions

Discussion in 'Transmission & Driveline Topics' started by Gasoholic, Feb 8, 2020.

  1. Burd

    Burd Veteran Member Lifetime Gold Member

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    Look on the tube. Code is stamped, my 400 has a 2:56. Gotta go by the code on the tube.
     
    Last edited: Feb 12, 2020
  2. 67johnny

    67johnny Veteran Member

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    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!;)
     
  3. Eddie Akers

    Eddie Akers New chapter

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    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
     
  4. badazz81z28

    badazz81z28 Veteran Member

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


     
  5. Kevin McKeown

    Kevin McKeown Moderator Lifetime Gold Member

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    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!
     

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  6. Fbird

    Fbird Veteran Member Lifetime Gold Member

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    simple 2 series posi carrier diff with useless gears $200 all day long...
     
  7. badazz81z28

    badazz81z28 Veteran Member

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

    big gear head Veteran Member

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

    Fbird Veteran Member Lifetime Gold Member

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    thought 2 series was 2.56 and 2.73.......then 3 series stared at 3.23
     
  10. big gear head

    big gear head Veteran Member

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    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.
     

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