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Tire Size Calculator Wrong?

Discussion in 'Wheels & Tires' started by nermalgod, Dec 6, 2010.

  1. Dec 6, 2010 at 10:35 AM

    nermalgod [OP] Well-Known Member

    May 17, 2008
    Madison, WI
    Offroad boat hauler
    Not going to brag.
    Specifically the revolutions per mile.

    I've looked a few different places and at specific tires. Often they list a RPM for the size which is considerably different from what the tire calculator spits out.

    When I do the math myself, it seems the tire calculator reads low by 20-30 RPM.

    For instance, BF Goodrich 265/70R16 according to tirerack.com spin 680 RPM. The tire calculator puts that size at 659 RPM.

    For General Grabber AT2 235/85R16 tirerack.com claims 652 RPM but the tire calculator claims 636 RPM.

    Has anyone else noticed this? I'm trying to estimate the change to my speedo/odometer, but numbers are not working when I use this tool.
  2. Dec 6, 2010 at 10:50 AM

    98tacoma27 is gooder 'en chicken Moderator

    Dec 18, 2008
    First Name:
    Beech Creek
    05 Tundra SR5 (+250k AND COUNTING), 88 SR5 PU (SOLD), 98 TRD OR (CRUSHED)
    5100's all on fours
    Using those calculations, that would make the BFG a 29.675" diameter tire and the AT2 a 30.94" tire.

    It appears the tire calculators is formulated to use advertised sizes and not actual sizes. Perhaps Tire Rack measured (or was supplied actual sizes from the manufacture) every brand and size tire for their database.
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