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Thread: Tire Size Calculator out by 5%?

  1. #1
    Lifetime Member Mitco39's Avatar
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    Default Tire Size Calculator out by 5%?

    Has anyone else been having this problem lately? We been setting the tire size to the customer supplied size and on 3 or 4 different occasions now it has been out by 5%. I dont have the trucks on hand so I couldnt confirm the actual size of the tire (I asked them to check, waiting to hear back).

    One customer has his tire size set to 31.75" with a ppe yet we set it with the efilive calc and it was 31.7, so it all seems good, but yet they are saying it is now out by 5% and the tire size we supplied with that adjustment is 30.2". The rear differential is 3.73s and confirmed.

    I find it just so odd that the PPE would be happy at that OD and efilive needs a different one. Not sure how that would even be possible, that said I have not confirmed this in person, only going off what I am being told.

    Just curious if anyone else has seen this on their day to day work? Seems odd.

  2. #2
    Lifetime Member THEFERMANATOR's Avatar
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    I always do it by tire height, or I look up the tire, and get the rev's per mile from the manufacturer to plug in. The calculator has always been off for me.
    1995 GMC 2500 SUBURBAN powered by 01 DURAMAX/ALLISON, little of this, a little of that,
    DIAMONDEYE 4" exhaust, CORSA muffler, AFE stage 1 dry filter, custom tuning by me, KENNEDY single pump and pump rub kit.

  3. #3
    Lifetime Member GMC-2002-Dmax's Avatar
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    I always ask for an exact measurement
    www.mscservices.net


    Tuner of many, many Duramax and Cummins Diesels.


  4. #4
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    It's been my experience that the factory calibration is always off a little bit. I figure it was GM's way of a fudge factor to make it work out. When doing tire size changes, I always start out with the GM calibration value (already fudge factored) and do the relative size correction. So far, this has worked everytime.

  5. #5
    Lifetime Member Mitco39's Avatar
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  6. #6
    Lifetime Member LB72004's Avatar
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    I had to use what the manufacturer says for revs per mile. it is about 1/2 MPH off according to GPS
    2008 Chevrolet 3500HD DRW LTZ
    285/75/17 Nitto Dura Grapplers, 4.56 gears, Front and rear ARB lockers, Efi Live tuning by me, Raptor 160 LP, Cognito 4-6 NTB Lift, 4" Exhaust, Built Trans by Mike L.

    Sold - 2004 LB7 Duramax, EFI Live with DSP5 Tune
    stage 4 SC transmission, 265/70/16 tires

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