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Technical 8.5" 10 bolt with 2.56 Gear ratio help

Discussion in 'The Hokey Ass Message Board' started by 49ChevTruck, May 22, 2016.

  1. Hey guys I have a 48 Chevy fleetline with a 350/350 combo in it, I have a 8.5" 10 bolt in the back with 2.56's open drive. I'm running a 28.88" Tire on the back

    At 70mph i'm doing 2000rpm....I took it up to about 3500rpm the other day on a long stretch of road and i'm pushing 120mph. In reality my car will do 170mph doing 5000rpm.

    Only problem is with it geared like this is that it's slow off the line. I was thinking about changing to 3.42's. Was wondering if any of you guys have made that big a jump and what's it feel like? Any opinions would be greatly appreciated.

    I couldn't give a shit about gas mileage so i'm not worried about the mpg it's not an everyday driver it's after work and weekend car.
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 47,769


    It will make a huge difference in how it feels.

    If you won't spend much time on the open road, going fast, then the 3.42 should work fine. Beware there might be different differential carriers for different ratios, because of the different pinion gear diameters. Looks like there is a cutoff between 2.56 and 2.73 ratio carriers. Make sure you get the right gears! or a new differential carrier or complete differential to match the gears.
  3. I'm going to get a new posi carrier, ring and pinion and all new bearings. I don't do many highway driving in it only to car shows. The future plan will be to get a gear vendors overdrive unit to put behind my 350.

    I'm sick of not being able to lay rubber!
  4. stimpy
    Joined: Apr 16, 2006
    Posts: 3,547


    if your thinking about a gear vendors , do the best of both worlds , a 4:10 for real get up and go in the rear then when you pop into over drive its a 3:20 gear so its not a lame duck , or minimum a 3:55 set and it will drop you in to the 2:70's range which is a decent cruising gear and will not lug the motor as much as a 2.50 set

  5. The gear vendors won't be for awhile, the 4.10's would probably be a little too much for the time being wouldn't they?
  6. I just did this swap in my non Hamb friendly 68 Camaro. I was running 2.73 gears in and open rear. I swapped in an 8.5 rear from a 72 nova that I upgraded to a posi carrier and 3.42 gears.
    The seat of the pants feel is great but on highway fuel mileage is reduced and the rpms have increased by about 650 rpm.
  7. squirrel
    Joined: Sep 23, 2004
    Posts: 47,769


    Depends. I drove Drag Week last fall with 4.10s in my Chevy II. Then I put a GV overdrive in it, and put 3.89 gears in, it's nice. 3.73 would be good with the overdrive, if the engine is pretty mild, and it would not be too bad before you get the OD.
  8. Hackerbilt
    Joined: Aug 13, 2001
    Posts: 6,247


    Just changed my T to 2.73 ratio from 3.43.
    Thats as high as you can go with the 3 series carrier...IF you were reusing it.
    I just didn't need 3.43's with the T350/454 combo, and I'm hoping the change gives my car some improved highway manners and better fuel mileage.
    Haven't had it out yet...but hopefully later this week.
    I doubt acceleration will suffer much on the road...LoL
  9. gnichols
    Joined: Mar 6, 2008
    Posts: 10,909

    from Tampa, FL

    Would putting in a GM 4 spd auto in now be better in the long run ICW your rear gear change? Just wondering, as I heard that those aux overdrives are kinda pricey. Gary
    Hnstray likes this.
  10. I was thinking about that but I like the look of the gearvendors unit, you basically get an overdrive for every gear. Plus later on down the line i'm going to be putting a sbc 427 in it, and i'm pretty heavy footed! I don't think a 700r4 would withstand the punishment i'm going to throw at it.
    Last edited: May 22, 2016
  11. Mr48chev
    Joined: Dec 28, 2007
    Posts: 28,341


    What I am seeing from the others is that the housing you have won't take 3.????? gears. As Jim said in post #2 the break for what you can run is between the 2.56 and 2.73 The design of the housing changed enough so that the gears won't interchange. There may be some aftermarket sets that work though.
  12. Hackerbilt
    Joined: Aug 13, 2001
    Posts: 6,247


    It's the differential carrier/Posi unit etc that is different.
    The main housing, axle tubes and all that stuff remains the same as far as I know. Can't imagine GM making a special main housing just for 2.56 ratios.
  13. It will take 3+ gears with a new differential and new gears to match....
  14. What Bill said.... it's exactly the same housing just a thicker flange on the 2.56 differential.

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