What should be the width of my new rear end?

Discussion in 'Got gears?' started by ravensbud, Dec 29, 2018.

  1. ravensbud

    ravensbud Silver Level contributor

    I'm looking to put a new rear end in my 67 GS400 that can handle drag strip launches with slicks. I'll be replacing the original BOP 8.2 before it explodes and I have to find another set of unobtainium axle bearings. The thing that has always bothered me about this rear is that it seems too wide for the car. Its 61" from flange to flange (the standard width from 66-72 I think) and using the diagram below, A=7" and B=4.5". This has always limited how wide a wheel/tire package I can fit in the wheel well. I've had to max out the backspace on an 8" wheel to clear the fender lip while leaving a bunch of unused space on the frame side.

    So the question is why did the factory make all these rears so wide? Wouldn't the ideal width of a rear end be one where the wheel mounting surfaces are exactly centered in the wheel well (i.e., A=B)? And while planning out my new rear end, shouldn't I be looking to have A=B=5.75" so its centered up in the wheel wells? This would mean my rear end should be 2.5" narrower, or 58.5" axle flange to axle flange. Thanks for any insight here...this is the first of a few questions I've come up with so far in the process.

    wheel measurement.jpg
     
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  2. BrianTrick

    BrianTrick Brian Trick

    The 71-72 8.5” 10-bolt will be the same width as what you have now. I’m sure BOP had some sort of reasoning as to why they went with the wider rear before Chevy did.
     
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  3. monzaz

    monzaz Jim

    Reason - The wider the rear the better the car handles corners... Wider foot print of the axle out ward.

    Look at the 1994 - 2002 camaro it is a 65" wide rear. The rims are 7" backspacing. Cuts the wind at higher speeds and when turning the corner the wider axles to the edge of the body prevents roll and added with the pan hard bars and sway bars it could really turn corners for the year the car was and price and technology.

    So GM had the Pontiac WIDE TRAC commercial way back in the gasser era...Pontiac was always performance oriented olds had the narrower rear and pontiac the wider rear in 1958 era even. Pontiac did this again in the Fire bird and camaro rears... 1967 firebird used the GTO rear which was 55.25 or 61" over all and the camaro was 54.25 or 60" over all. WIDE TRACK commercials again
    GTO did it too. 1966 they had the 55.25 rears also ...
    Remember too the wheel and tire technology was not very good. g70 was about the best wide low profile you could get from the factory not even sure when that optional size started

    SO when you have only skinny little tires - it was not a HUGE concern and they did center the tire/ rim with the size tire they had for options. 4.25 rim backspacing was stock gm back then.

    I personally bought rally Chevy rims for my 1971 monte carlo because i wanted 295 50 15 so I ordered 4.5" rim back spacing to center that tire under the rear wheel well.

    IF you like the rear......... too bad you were not closer we could cut those ends off and install stock Pontiac ends and you could even narrow some at least another 1/2" to maybe 1" on each side. Jim

    J D Race
     
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  4. ravensbud

    ravensbud Silver Level contributor

    Well I'm glad I asked because your explanation makes total sense. The factory's goal was handling and safety, my goal is to fill the wheel well with all the rubber I can. And doing the math...4.25" backspace on a 6" rim does center the tire in the wheel well on my car. Cool.

    I'm still not sure what to do about the width of my new rear end. An 8" wheel is about the widest I'll ever be able to use on this car, and 5.25" backspace would center it with a 61" wide rear. Seems like that's usually a custom backspace order, whereas shortening to a 60" overall width (1/2" shorter on each side) would allow for
    4.75" backspace which seems to be more easily obtainable on an off the shelf wheel.

    I've been leaning toward a Ford 9" bolt-in aftermarket rear that I could specify the width on, but perhaps a GM 8.5 narrowed a bit would be the better option. I'll give you a call to discuss, Jim.

    Now for anyone who'd like to see a vintage Pontiac Widetrack commercial, enjoy:

    <iframe width="560" height="315" src="" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></i
     
  5. 455monte

    455monte Well-Known Member

    I built a 64 buick special 8.2 for my 67 gs.
    Its narrower than 66 up housings.
    I run a 15x8 with 4.5 bs and a 275 60 tire.
    Since my fender lips were trimmed for a previous set of wheels also i believe i could easily get away with a 295 tire as there is still room left on both sides
     
  6. monzaz

    monzaz Jim

    I sell tons of Moser 12 bolt 65-67 NEW rears 2350.00 that are 60" wide.
     
  7. monzaz

    monzaz Jim

    All your brakes will bolt right up.
     
  8. BrianTrick

    BrianTrick Brian Trick

    Jim is correct about why they went wider. I am just curious why Chevy took another 2 years to go to the wider rear. They must not have gotten the memo.
     
  9. monzaz

    monzaz Jim

    Chevys were the LOW grade car. Lowest line of GM
    It has always been that way. BOP always got better gears , More cubes, better ride better insulation... Always. Chevy , Pontiac, Olds, Buick , Caddy that is the list in order of importance.
     
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  10. BrianTrick

    BrianTrick Brian Trick

    BUT Chevy is the last to get the axe from the boss.
     
  11. monzaz

    monzaz Jim

    You need economy and you need luxury... too many in between cars. Buick was the choice I guess. ??
     

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