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Racing Tire Sizes

Discussion in '348/355' started by vvassallo, May 19, 2010.

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  1. vvassallo

    vvassallo F1 Veteran

    Aug 4, 2006
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    Vince V
    #1 vvassallo, May 19, 2010
    This might be an obvious point but the stock tire size for a 355 is 225x40x18 front and 265x40x18 rear. However, racing tires come in 245x645x18 front and 305x645x18 rear. What is the 3-digit second number for?

    I guessed that it is a total measurement of something (overall tire height?) that is divided into the first number to get what we are used to seeing which is the profile expressed as a percentage. Hence the 1st example is a 245x38 (245/645) x 18. Anyone know this?
     
  2. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    9,631
    #2 Mitch Alsup, May 19, 2010
    It is the rolling diameter: 645mm = 25.39 inches.

    WIth the (ahem) standard way to denoting this:

    225/40ZR18 -> (18*25.4) + (225*0.4*2) = 637.2 = 25.08 inches
     
  3. WallaceBow

    WallaceBow Karting

    Jul 13, 2008
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    Wallace Bow
    #3 WallaceBow, May 19, 2010
    Last edited: May 19, 2010
    You must be looking at the tire size for Pirelli racing slicks. The second number is the approximate circumference of the tire in millimeters. See this web site for more information on racing tire sizes and choose Pirelli and P Zero Slick:

    http://www.bobwoodmantires.com/bwttires.php

    This web site has actual circumferences and widths for the different tire sizes. Don't be fooled by the 645mm circumference, the four Pirelli tires listed as 645s have circumferences of 649, 650, 655, and 654mm respectively for widths of 245, 265, 285 and 305mm.

    I have a set of Hoosiers for my F355, 225x40x18 front and 275x35x18 rear. The 265x40x18 is an oddball size and few manufacturers make it, even in street tire form. I am very happy with this size combo, though. I don't have a lot of experience driving mid-engine cars and the 275x35x18 rear seems to make the car less tail-happy. The downside is that the 275x35 is slightly shorter than a 265x40 so I will likely look for a set of 19" wheels for future race tires. Anyone else?

    Wallace
     
  4. vvassallo

    vvassallo F1 Veteran

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    #4 vvassallo, May 20, 2010
    Wow, thanks for, ahem, clearing that up. I vaguely understand now. It's for a measurement, that is an approximation, of dragging a tape measure around the tire. :) And yes it was for the racing tires, both Pirelli and Hoosier.
     
  5. gothspeed

    gothspeed F1 World Champ

    May 26, 2006
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    #5 gothspeed, May 20, 2010
    Last edited: May 20, 2010
    Just thinking on the surface .......... if both the fronts and rears are designated with the same 645, that number is probably not the circumference. Seeing as how our cars almost always have a lower profile front vs rear. A smaller profile in front with the same wheel size, will definitely equal a smaller circumference.

    BTW Vince ......... when will you be ready for your 285/35-18s rears I have for you :)?
     
  6. WallaceBow

    WallaceBow Karting

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    #6 WallaceBow, May 20, 2010
    Last edited: May 20, 2010
    Gothspeed is clearly right and I can't read, apparently. 645mm is the approximate DIAMETER of the tire, not the circumference. The website also clearly shows that, I just overlooked it. I'm going to edit my original post to make this thread even more confusing.

    Uh oh, it won't let me edit the previous post. Now I will look like an idiot on the internet for all of eternity which I'm pretty used to by now. Let's see if we can do the math on the 245x645x18 and see if we can compute an aspect ratio for comparison to the 245x40x18 OEM tire size:

    If the DIAMETER of the tire is 649mm (from the website), then the amount of sidewall is 649mm-18in*(25.4mm/in) = 191.8mm. Of course there are sidewalls on each side of the rim or else your tire rides all lumpy, so that means that one side wall is 191.8mm / 2 = 95.9mm. From the website, the actual treadwidth is 240mm, so the aspect ratio of the tire would be 95.9mm/240mm = 0.3996.
    That's your 245x40x18 equivalent, right there.

    Let's run the numbers on the 285x645x18, too. The actual diameter is 655mm so 655mm-18in*(25.4mm/in) = 197.8mm. 187.8mm / 2 = 98.9mm sidewall and the treadwidth is 280mm so the aspect ratio is 280mm/98.9mm = 0.3532. This tire is a 285x35x18 equivalent.

    Sorry for the confusion,

    Wallace
     

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