can't decide if its worth it 2.73 open to 3.42 posi

Discussion in 'Transmission & Driveline Topics' started by 351maverick, Sep 11, 2019.

  1. 351maverick

    351maverick Veteran Member

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    in my 3rd gen 88 Firebird Formula

    currently has a one wheel peel 2.73 10 bolt

    a bunch of local guys have 3.42 posi 10 bolts out of T56 6 speed 4th gens ready to bolt in for less than $300

    my 18" rear tires are getting old & hard...one will often spin not even laying into the gas much

    4th gen rears are basically a bolt in, and it would eliminate my 2" wheel adapters in the rear

    2.73 to 3.42 isn't that big of a difference...I'd much rather 3.73 or even 4.10 (700R4) but nowhere near as cheap as a factory assembly
    20180813_201148.jpg
     
  2. Coadster32

    Coadster32 Veteran Member Lifetime Gold Member

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    I'm thinking that you have more cars apart than anyone else on the board on a daily basis, so the effort shouldn't be too great for you. For the price, this seems like a win to me. I understand about the gearing not being that different. If you don't need posi, and you want the taller gears, then hold off, and sell some more car parts till you raise up the cash. Perhaps get the other rear, and put in new gears before you install it?
     
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  3. big gear head

    big gear head Veteran Member

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    The 3.42 is usually pretty much perfect for a 3rd gen. The 700R4 transmission has a 3.06 first gear which is usually too much when going to a 3.73 or 4.10. The 4th gen width seems like it will be perfect for your wheels. I think it's a good swap.
     
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  4. ZEN357

    ZEN357 Veteran Member

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    I think it would be a good swap since it's an automatic transmission. When I owned my 91 Z/28 I wen to 3.73 in the rear but I had a 5-Speed. Made a world of difference. I had a friend do the swap at the time and install a locking rear. He only charged me $100 to do the swap and I helped him so I would learn how to do it.
     
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  5. xten

    xten Veteran Member

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    I'd try it Mav. $300 is cheap. I think that would be a big bang for the buck. Besides, the 3:42's with that 3:06 first gear will feel pretty good. Had 4:10's with a 700R and first gear was useless. Went to 3:89's (9") and wish I would have gone more shallow.
     
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  6. Bandit723

    Bandit723 Veteran Member

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    I recently went from 3.73's to 3.42's (700R4) definitely made the 1st gear feel much better.
     
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  7. biker

    biker Veteran Member

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    Definitely worth it.
     
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  8. 351maverick

    351maverick Veteran Member

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    you guys are inspiring - thank you
     
  9. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    That's the swap I made... 2.73 peg-leg to a 3.42 Eaton posi. (The 25% difference in rear gear ratio is quite noticeable.)
    I also installed a 2004R (0.66 OD) transmission so the final ratio in OD is around 2.25 for highway cruising. (2000 RPM @ 70 MPH)

    The 7004R has a similar OD ratio (0.7) so I would expect around 2100 RPM @ 70 MPH with 3.42 gears.
    Downshifting (via throttle/TV cable) is much more fun with the 3.42 gears. ;)
     
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  10. Zspoiler

    Zspoiler Veteran Member

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    3:42`s are a good compromise gear .You get good acceleration and decent gas mileage. I have a set in both my 1979 -28 with a 4 speed supercharged 377 small blok and a 1981 El Camino SS 350 small block turbo 350 .and both get around 22-5 MPG. with a 26 inch tall tire at 2500 rpm
     

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