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Wheel diameter question

This is a discussion on Wheel diameter question within the Wheels and Tires forums, part of the General Help category; For the purposes of using MPH/RPM calculations, how do you determine the diameter or circumference of your wheel? I have ...

  1. #1
    230,55,147 91Z28's Avatar
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    Cyclone Grey Metallic
    2006 GTO

    Wheel diameter question

    For the purposes of using MPH/RPM calculations, how do you determine the diameter or circumference of your wheel? I have 40/18s.

  2. #2
    Crazy Canuck ! Eugenio_SS's Avatar
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    triple-black
    2000 CamaroSS convertible

    assuming it's a 275/40-18
    40 means sidewall is 40% of 275mm. = 110mm
    you have the wheel + 2 sidewalls for the overall diameter.
    2 sidewalls = 220mm = 8.66"
    18" + 8.66" = 26.66" which is 1" more than stock.
    stock is 25.66"..., so your speedo is off by 3.9%
    you are going 4% faster than the speedo shows.
    Eugenio_SS
    almost stock triple-black 2000 SS convertible with 17x11s on all 4 w/ 315s at the track or on the street with 18x10.5s on all 4 w/ 315s: (1), (2)

  3. #3
    230,55,147 91Z28's Avatar
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    Thanks

  4. #4
    1KBird
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    Quote Originally Posted by 91Z28
    For the purposes of using MPH/RPM calculations, how do you determine the diameter or circumference of your wheel? I have 40/18s.
    Check this out. 1010tires.com/TireSizeCalculator.asp Don't forget the www up front. I can't post full links yet.

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