Gear ratio, wheel size, and TH400

Discussion in 'Got gears?' started by matt68gs400, Aug 31, 2015.

  1. matt68gs400

    matt68gs400 Well-Known Member

    Hey guys,

    Im thinking about the best wheel size, differential ratio, etc that would be a good fit for me on my 1968 GS. Right now here's what I have:

    -455 from a 71 Buick with a mild cam. Not sure what else had been done, probably not much. Thinking upper 300 HP,highly unlikely it reaches 400.

    -turbo hydramatic 400 transmission

    -Chevy 12 bolt with about 3.31 gears based on jacking the car up on one side and rotating one tire two revolutions. The drive shaft turns a little over 3.25 times.

    -open rear end! It's a one wheel wonder that can't get off the line.

    -BF Goodrich Radial TA, P235/70SR14.

    -3200 rpm at 70 mph

    i want street ability, quickness, and to be able to drive 3 hours if needed. So I'm looking at a compromise. A 4.10 rear is out of the question.

    Other than a posi or Eaton, what else should make a big difference? I'm wondering if I should go to 15x8 rims. I want to keep the stock looking rally rim.

    i first want to do posi, but want the right gear ratio. I'd also consider wider tired of the car allows it. The car doesn't catch off the line at all as you can imagine.

    Thanks!
     
    Last edited: Sep 7, 2015
  2. Oldskewl59

    Oldskewl59 Gold Level Contributor

    I have a 71 GS 455 that is pretty much stock and you do want as much tire as you can get at the rear. You can get a 15 x 8 rally wheel. You also want the posi. Mine is a 12 bolt with 3.55 gears. It is a little much for the highway and it gets poor gas mileage, but I do not drive on the highway with it that often. It makes the stock motor really come alive at a low RPM and feels like a muscle car should.
     
  3. matt68gs400

    matt68gs400 Well-Known Member

    Thank you! Will a 15x8 fit in a 1968? I thought I read that the 70-72 have bigger wheel wells?
     
  4. bobc455

    bobc455 Well-Known Member

    So you have a fairly mild engine, but it seems like you want to put a lot of emhpasis on drag racing.

    I had a 2.56 rearend that could run 11's, but could also do the 3-hour drive with absolutely no problems.

    My gut reaction is that you want a 2.56 for the 3-hour drives, and if you run the numbers I think you'll find that it's only hurting your 1/4 mile time by about 0.1 second compared to a 3.42. How important is the 1/4 mile to you?

    -Bob C.
     
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    That is correct. In order to use a 15 X 8" wheel in the 68-69 cars, you need more back spacing. With an 8" wheel, the 70-72 cars need 4 1/2" of back space, the 68-69 want 5".

    I'm not real clear on what you are actually asking in the 1st post. Higher numerical gearing revs the engine higher the faster you go. Gears are multipliers of torque. The faster the engine revs on the highway, the more gas you burn. It's a trade off unless you use an overdrive. What are you willing to live with? What are you willing to give up? The stock tire was 26.6" tall, so anything higher than that decreases the numerical effective gearing. I have 3.73 gears with a 28" tire. 26.6/28 X 3.73 = 3.54.

    Your current tires are nearly 27" tall. RPM = 336/tire height X rear gear X MPH. Using your numbers, 336/27 X 3.30 X 70 = 2880 RPM with no converter slip, figure 3000 RPM.

    I think 3.08 gears is the perfect compromise. In the end, you must make that decision for yourself.
     
  6. matt68gs400

    matt68gs400 Well-Known Member

    0.1 gain isn't worth it to me. Thank you!
     
  7. matt68gs400

    matt68gs400 Well-Known Member

    So does the 68 want or does it need 5" back spacing? I see lots of 8" rally wheels for sale with 4.5" back spacing. Haven't seen 5", but didn't look that hard yet.I definitely want the car to catch rather than spin one wheel, so posi is a must in the next year. I'm just not sure what to do with the differential gears and tire size. I'm willing to sacrifice 0.2 seconds In quarter mile. The drag racing is just something to do a couple times a year.
     
  8. BrianTrick

    BrianTrick Brian Trick

    The wheels generally came centered under the car from the factory,so any time you go to a wider wheel,you should go inboard as much as you go outboard.
    as for the gear ratio,it sounds like your 12-bolt Chevy has the 3:31. I would leave that in it and add a posi.
     
  9. matt68gs400

    matt68gs400 Well-Known Member

    Thank you. I think that's what I need to do. Just get the posi or locker and then go from there. Are there 3.25 or 3.30 hears for that rear end? Not that I want them, I just know that turning the wheels only gives me a certain accuracy.
     
  10. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member


    I meant that if you want to put the widest tire you can on the 15 X 8 wheel, in your 68, the wheel must have 5" of back spacing. You will need to have them made as they come most often with the 4 1/2" B.S. Alan Wander (69gs400s) has a P275/60R-15 on an 8" wheel on his 69GS400. I would leave your present gears and just add a Posi. You can alter the effective gear ratio by altering tire size. A 28" tire (P275/60R-15) will make the 3.31's like 3.14, and a 26" tire (P275/50R-15) will make it like 3.38.
     
  11. BrianTrick

    BrianTrick Brian Trick

    The gear ratios available for 12-bolt Chevy are 3:07,3:31,3:42,3:55,3:73,and up from there. The 3:31 was a GM gear,but has recently been available in the aftermarket.The 3:42 is an aftermarket ratio.The 3:07,3:55,3:73 were available from GM,and also available aftermarket.
    You can go onto Randy's Ring & Pinion website,and go to the Gear Calculator. Enter your gear ratio,tire size,and transmission,and it can give you rpm ranges & such.You can play around with ratios & tire sizes on there.
     
  12. matt68gs400

    matt68gs400 Well-Known Member

    Guys, I really appreciate your knowledge and help. This forum and the people on it are a true blessing. It helps me pursue my addiction, err I mean hobby. I think I have gears that I can work with so I'll leave them. Should I be looking to find a GM posi or an Eaton?
     
  13. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    I have an Eaton, works great.
     
  14. BrianTrick

    BrianTrick Brian Trick

    The originals were an Eaton. I would just buy a new one. Also known as the 3-series posi,for 12-bolt Chevy.
     
  15. 70Cat

    70Cat Well-Known Member

    Put narrow 28" tires on your current rims for highway cruising and get 2 15x8 wheels with a 26" drag tire for the track. A 15 minute tire change and you're set for whatever you want to do...once you put a limited slip in there.
     
  16. matt68gs400

    matt68gs400 Well-Known Member

    do I need 5" BS for 8" rims on just the back wheels?
     
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    More so on the back wheels. If you plan on 275's, you'll need 5" of back spacing. The fronts will likely be narrower and you can get away with stock 7" wide wheels with stock back spacing. Every car is different, especially in the back. You MAY have more room on one side than the other. There are no guaranties.
     
  18. matt68gs400

    matt68gs400 Well-Known Member


    Thanks, I'll have to try contacting Alan Wander and get his thoughts. I'm hoping I can get away with 4.5" BS and maybe something a bit narrower than 275s on an 8" wide rim, unless I have to go significantly narrower.

    any thoughts on what's the widest I can go with a 7" wide rim?
     
  19. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Alan may be able to advise you further. I can't say absolutely what will fit.
     
  20. BrianTrick

    BrianTrick Brian Trick

    There are 255's
     

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