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Technical 37-56 Pontiac rear axle ratios

Discussion in 'Traditional Hot Rods' started by AGELE55, Mar 17, 2021.

  1. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    My 39 Poncho has a stock 4.11 rear axle. From what I understand, Pontiac ran the same rear end from 37 to 56. Does this mean I can swap in any pumpkin from these years to get a taller gear and reduce rpm at highway speed?
     
  2. Bird man
    Joined: Dec 28, 2009
    Posts: 709

    Bird man
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    from Milwaukee

    yes. Olds too. (I think)
    Seems like '56 might have used a different U joint flange?
     
  3. Im thinking you have to drill out the stud mounting holes to the Pontiac size to do this.........but I may be wrong.
     
  4. Bird man
    Joined: Dec 28, 2009
    Posts: 709

    Bird man
    Member
    from Milwaukee

    If swapping a R&P, Hollander sez because of bearing differences, Ya'll need a 39-47 R&P.
    If using the whole drop out, Olds & Poncho 39-55 will pretty much bolt in.
     
    AGELE55 likes this.

  5. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    Awesome....now all I need to do is track one down. Hmmm..
     
  6. jimmy six
    Joined: Mar 21, 2006
    Posts: 10,267

    jimmy six
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    Gevo’s rears and gears in New York. Found me a set 2.56 Pontiac’s..
     
  7. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    . A 2:56? My fuzzy math says at 75 mph , assuming a .70 OD tranny, I’d be turning somewhere around 1500 rpm. Definitely a cruising gear.
    Looks like the 2:56 is a 57 and up.... damn, I was digging on it.
     
    Last edited: Mar 17, 2021
  8. Frank Carey
    Joined: Oct 15, 2009
    Posts: 538

    Frank Carey
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    I put a 1940 Pontiac rear in a Model-A about 1960. I remember it has preload pinion bearings. I doubt that was continued into the 1950s but the drop outs might be interchangeable. Check an old Motors Manual.
     
  9. jimmy six
    Joined: Mar 21, 2006
    Posts: 10,267

    jimmy six
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    Skinner 12 port head and injection on a 52 GMC 6, 32” tire, 78% nitro in the tank, 32 Ford grille on a Highboy roadster, 2.56 rear gear, 2 way average of 210.9 mph = a Bonneville 200 MPH Red Hat..
     
    AGELE55 likes this.
  10. I've encountered 4.30, 3.90, 3.23. I'm sure there must be some I've not personally encountered. Last one I did was removing a 3.90 [called the economy gear for '41] third member from my 41 coupe and used a '56, 3.23 gear. I had to use the '41 pinion flange on the '56 gears but it was a straight across swap.
    Olds is the same but Olds used a different spline on the axle shafts'. I also swapped a 1948 pontiac 3.90 out for a 1955 Pontiac 3.23. Sure helps on the interstates. Photo shows the '56 third member with the modern pinion flange before I swapped it, 41newthirdmember.jpg
     
  11. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    Interesting... I have not found any reference on line to a 3:23 in a 37-56 Pontiac or Olds. If I could locate one, I think it would be a great solution.
     
  12. The 3.23 ratio is the standard ratio for 55-56 Pontiacs having the 4 speed hydramatic trans. I've heard there was a 3.07 ratio too but I've never seen one.
     
    AGELE55 likes this.
  13. jimmy six
    Joined: Mar 21, 2006
    Posts: 10,267

    jimmy six
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    57 was the start of rears getting higher gear ratios. Engines or 300” were the norm except for Chevrolet and highway speed limits began moving up. Most manufacturers were putting ing stronger rear axles again with exception of Chevrolet because hp in the average car was over 200.
     
  14. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    I’ve been looking at http://www.classiccardatabase.com/postwar-models/Pontiac.php and the tallest gear I see for the 55-56 Pontiac is a 3:64. I’m heading to Daytona for the spring Turkey Rod Run this weekend. Maybe the Rod God will shine on me and lead me to a suitable rear gear.
     
  15. One of my 57 Pontiacs had a 3.64 gear in it but it was born with a 3 speed manual and the "economy" low compression, 2 barrel motor. It needed the lower gear because those select shift 3 speeds were initially made back in the 30s when the cars had gearsets in the mid 40s like 4.30 to one. First gear was hellaciously high and that was ok, using the the low rear end gear of the 30's-40's but by 55-56 the most common rear gear was 3.23. Couple that with a 3 speed with ultra high first gear and those heavy cars [3600 lb] were clutch eaters! [I replaced 4 in one summer] A guy had to slip hell outta
    the clutch to get going from a stop and if you were stopped, facing uphill and you had a few buddies in the car, those little 9.5-10 inch clutches didn't have a chance!
    I swapped a 389 and a Muncie wide ratio into my first 57 and with the 3.64 gears, I gave those 389 GTO's fits. 57Ponchoonstreet.jpg
     
  16. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    So today I encountered a Pontiac/Olds differential which I believe is a 63- 64 ?
    The casting numbers were F203, and below that 543317. I didn't buy it cuz I don't think it will fit a 39 housing at all, but the the guy only lives an hour away.
    Any chance it will fit?
     
  17. Bird man
    Joined: Dec 28, 2009
    Posts: 709

    Bird man
    Member
    from Milwaukee

    No chance.
     
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  18. Beanscoot
    Joined: May 14, 2008
    Posts: 2,193

    Beanscoot
    Member

    Jimmy Six, I'm curious where you got the 2.56 ratio diffy from.
    I like the idea of a very low numerical rear end combined with a four speed with a high numerical first gear. Does the same as an overdrive but with a stronger, more durable 1:1 high gear.

    Edit: Doh, I see you already told us at the beginning the gear set was from "Gevo's". Still, I'm curious that such a gearset was ever made for this diffy.
     
    Last edited: Mar 30, 2021
  19. AGELE55
    Joined: Jan 4, 2018
    Posts: 398

    AGELE55
    Member

    Yeah, I know. Just hoping for a miracle..lol. Thanx
     
  20. 48fordnut
    Joined: Nov 4, 2005
    Posts: 4,088

    48fordnut
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    I used a 39 pont rear chunk in a 50 olds, 3.90 with a cad /lasalle box.
     
  21. Frank Carey
    Joined: Oct 15, 2009
    Posts: 538

    Frank Carey
    Member

    The ratio on this rear was 4:10.
     

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