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can someone explain?

Discussion in 'Tires & Wheels' started by rydinhigh, September 21, 2002.

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    1. rydinhigh

      rydinhigh Guest

      I am so curious and could never understand this. When they give measurements they say 275/55/r17

      I know that the last number is the size of the rim. what are the first two numbers for?
       
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    3. section525

      section525 sextion525 Elite Explorer Moderator Emeritus

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      Straight from tires.com

      A term that describes a tire's height-to-width proportion. If a tire's sidewall height were 65% of its cross section width, its aspect ratio would be 65. In the tire size expressed as 205/65-15, the number 65 is the aspect ratio.

      [​IMG]

      Basically, the 205 is width in mm (I believe). And the 65 means the height (not overall height, but sidewall height) is 65% of the width.
       
    4. Crankcase

      Crankcase Moderator Emeritus Moderator Emeritus

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      I think the 205 is sidewall height in MM. The sceond number is the aspect ratio of that . A 235 75 R15 tire is 10 MM HIGHER than a 225 75 R15.
       
    5. rydinhigh

      rydinhigh Guest

    6. JDraper

      JDraper Somewhat Functional Moderator Emeritus

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      Actually, the 205 is the tread width, not the height. The second number is the aspect ratio, which is the height of the tire sidewall as a percentage of the tread width. In your example above, a 235-75/15 tire has a sidewall height of 176.25mm (235 x .75) and the 225-75/15 tire has a sidewall height of 168.75mm (225 x .75).
       
    7. Howard

      Howard Moderator Elite Explorer Staff Member Moderator Elite Explorer

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      The first number 275 is the tread width the second number 55 is side wall height as percentage of tread width and the last number is rim diameter.
      So to work your example and convert to inches as follows:
      275mm or 10.8" tread width
      55% of 275 = 151.25 or 5.95"
      17" rim size so total wheel rim diameter = 5.95+17+5.95=28.9" try it and see.
      On the onther hand my standard tyres are 255/70/16 which equates to: 10" / 7" / 16" diameter 30" approx.
      Hope this clears up the confusion. To gain ground clearance you want the biggest diameter wheels that you can fit depending on lifts etc.
       

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