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Thread: Tire Sizes

  1. #1
    Join Date
    Feb 2001
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    68
    Can someone post a link for a tire calculator that tells you how much your speedo will be off..

    mike
    1993 325i
    Racing Dynamics Body Kit
    AA Cold Air Intake
    M5 17" Wheels

  2. #2
    Bob ///M3's Avatar
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    BIMMERFORUMS
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    Here you go Mike!

    You'll need the old and new tire size (diameter); the final (drive gear) ratio and to input a speed (mph).

    Speedometer Error Calculator

    Bob ///M3


    Originally posted by Mike325i
    Can someone post a link for a tire calculator that tells you how much your speedo will be off..

    mike

  3. #3
    Join Date
    Feb 2001
    Location
    CT, USA
    Posts
    379
    Actually you shouldn't need to include any gear ratio whatsoever as the answer is easier than you think. Let's say you go from 225 to 245. Take the 225/40-17 (24.09 in dia) wheel and the 245/40-17 (24.72 in dia) wheel; dividing new by old will result in about a 1.026 increase. This will be the factor your speedo will be off (reduced, so divide by 1.026) because everything else is equal and cancels out. To further explain, the 24.09 dia wheel is rotating 2630.14 times per mile, whereas the 24.72 dia wheel is rotating 2563.11 times. The fewer rotations that the larger tire completes means the speedo indicates a speed inversely proportional to the increase in diameter.

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