Big tire guys what gear ratio are you running?

Discussion in 'Got gears?' started by bignastyGS, Jan 14, 2021.

  1. bignastyGS

    bignastyGS Maggot pilot

    Getting closer to having the back half car to the point of purchasing a center section for it. I have 4 quotes from builders but they all have different gear ratio suggestions. 3 Strange centers vs 1 Moser. Gear recommendations from 3.89.4.11,4.56 and 4.88. Rear is a Ford 9 inch with a set of offset length 35 spline axles. Moser axles and 13 inch drum brakes.. The car is a 70 Buick that will be in the 3200 lb range hopefully. TH400 transmission and a 464 with Aluminum Stage 2 heads. 3 inch exhaust fed by TA Stage 2 headers. Tires are MT 31x18.50x15 on widened Buick rally rims 15x14. I am not planning of racing this car as it's going to be a more streetable car. No huge cam or such. Want to be a dependable ride to drive on Sundays etc. From all the gear ratio pages, I keep getting 3.89 gear selection. I am just hoping that it accelerates well and not be a slug, but I don't want to run 3000 plus at 70 mph. I'd rather keep the TH400 and am not wanting a Gear Vendor unit at this time. A 200 or 700 trans isn't going to happen either. The guy that suggested the 4.11 is really a good guy and stands behind his work. I have friends that he did work for. I just don't want the higher rpms like the 3.90 in my other car. He suggests a Detroit Tru Trac posi unit, Strange center section and I forget the gears name.Am I paranoid or will the 4.11 be too much? I know I have a tall tire,but it's eating at me..
     
  2. Stage 2 iron

    Stage 2 iron 472 IRON HEAD STAGE 2

    70 stage 1 The original owner took the 364s gears out and installed 430 gears back in the day. With a 27 -28 inch tire 40-45 mph at 3000 rpm. You Definitely need to pick the right cam and stall converter also.
     
  3. LARRY70GS

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

    Most gear ratios are based on a stock tire height of 26.6". Taller tires numerically decrease the ratio. To figure a final drive ratio, divide 26.6 by the actual tire height, and then multiply by the rear gear ratio to get final drive. 26.6/31 =.856

    .856 X 3.89 = 3.33

    .856 X 4.11 = 3.52

    .856 X 4.56 = 3.90

    .856 X 4.88 = 4.18

    336/tire height X gear X MPH = RPM (add 200 RPM for converter slip)

    So, for 3.89.... 336/31 X 3.89 X 70 = 2951 + 200 = 3151 RPM

    .............4.11......336/31 X 4.11 X 70 = 3118 + 200 = 3318 RPM

    and so on.
     
  4. Bens99gtp

    Bens99gtp Well-Known Member

  5. LARRY70GS

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

  6. Bens99gtp

    Bens99gtp Well-Known Member

    Yep if you look at the link under 3speed in 3rd, thats the same number, the auto figures in some converter slip, but since every converter is different that number is just a round about close estimate. I see too many numbers in my life so less math is good.......1 less thing I can mess up
     
  7. bignastyGS

    bignastyGS Maggot pilot

    So, I talked to the builder and went with his suggestions. But very similar to my research.
    Yukon Nodular case 3.250 bore
    Daytona Billet 1350 yoke
    35 spline Tru Trac Posi unit
    Motive Gear in 3.70 ratio
    New Timken bearings,small parts, gasket, and set up to go

    It will run about 2807 rpms at 70mph. I may decide later to get a Gear Vendors OD unit,but I don't think that rpms range it terrible to cruise at.The Nodular center is 20 lbs heavier than the aluminum one,which doesn't worry me..
     
  8. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    I would be surprised if those tires actually measured to 31"..
     
  9. bignastyGS

    bignastyGS Maggot pilot

    Yup...31 inches ...... 20210118_140625.jpg
     
  10. LARRY70GS

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

    Poor man's overdrive.:) Yardley uses 30.64" tires on his Riv when he drives long distance to Buick meets. They are P265/75R-15.
     
  11. bignastyGS

    bignastyGS Maggot pilot

    Larry, How much gear ratio do you lose with a Gear Vendor unit? Not worried about fuel mileage,but losing some rpms.
     
  12. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Usually the tires are no where close to their advertised diameter.
     
  13. LARRY70GS

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

    It's .78 Pat. So 3.70 X .78 would be 2.89.
     
  14. bignastyGS

    bignastyGS Maggot pilot

    Thanks Larry..I know of a near new GV setup sitting on a shelf that I am going to start bugging the guy...

    Joe, The advertised size is 31.0 but when I got to investigating,it was closer to 31.20 inches..They are kinda heavy with the Buick wheels too..
     

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