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Tire Size/ Speedometer Question

Discussion in 'Car Forum' started by TxTech, May 28, 2008.

  1. TxTech

    TxTech

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    On my Grand Cherokee I went from 235/65/17 to 245/75/17. A tire calculator I looked at said when my speedometer says 60MPH I am actually going 65MPH. Does this change throughout the speed range or am I always 5MPH off?
     
  2. knightkrawler00

    knightkrawler00

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    It will change throughout the speed range. The faster you go, the bigger the difference will be.
     

  3. IslandHopper

    IslandHopper

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    So when the car isn't moving, you're actually going 5mph... :upeyes:
     
  4. Bowtie

    Bowtie NRA MEMBER

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    Why is that? Do the tires get bigger as he goes faster?
     
  5. Ashton

    Ashton

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    If he's off by 5% of 50 MPH that's 2.5 MPH - at 100 MPH the variance would be 5 MPH. Yeah the added circumfrence will make the speedo appear slower than actual. measure the tires though - tire manufacturers are like shoes and jeans - actual measurements may vary - you may be better or worse off than you think you are - depending on the real measurements.

    Why did you up the sidewall? Add ground clearance??