Tire Sizing Questions

Discussion in 'Technical Q&A' started by Jim Riggs, Jul 19, 2004.

  1. Jim Riggs

    Jim Riggs Karting

    Jan 25, 2004
    I have a 348 SS that I have put a set of 355 Challenge wheels on. I bought a set of Bridgestone O3's [225-40-18 and 275-40-18] for these wheels. I noticed that the rear wheels have about a one-inch large rolling diameter than the stock wheels [215-50-17 and 255-45-17]. Before I buy a another new set of tires I wanted to fully understand the issues associated with selecting the proper tires sizes for this application. [I did a search on the subject, but I am still not clear].
    1. Given the tires size (e.g., 225-40-18) is there a formula that allows one to calculate the rolling diameter or do you have to use a table?
    2. I am told that you want to maintain as close to the same rolling diamter as the stock so as not to adversely change the suspension setup. How close is close enough and how much is too much?
    3. I was also told that you want to try to maintain the same difference in width from back to front as with the stock tires. How important is this factor or is it really important?
    I really want to make sure that I get the optimum handling setup for my F-car and I would sincerely appreciate your feedback on these issues. Cheers, Jim
  2. Frank R. Masiarz

    Nov 10, 2003
    Full Name:
    Frank R. Masiarz

    Check the Tire Rack site: for information on tire specifications. The site contains massive tables which summarize diameter and cross section width, both variables which affect tire clearance in the wheel wells and performance.

    There is a simple formula for the calculation of the diameter of a tire, given the size designation; don't have it right now.

    Check out: for calculation of speedometer error with changes in tire size.

    There are many books which provide information on tire and wheel selection and upgrades for performance. Check out:

    High-Performance Wheel and Tire Handbook by Jim Horner
    ISBN 0-87938-321-6 - paperback book in 1988
    Motorbooks International Publishers and Wholesalers

    Good luck !!

  3. stevehuck

    stevehuck Formula Junior

    Jun 4, 2004
    Sterling Hgts, Mi
    Full Name:
    Steven Huck
    Tire Height = (Tire Width X Tire Aspect X 2) / 2550) + RimSize
    Tire Width = Tire Width / 25.5

    225 Width
    40 Aspect
    18 inch Wheel or rim

    225 / 40 R 18 is a 25 inch tall 8.8 inch wide
    275 / 40 R 18 is a 26.6 inch tall 10.75 inch wide

    I can't remember the source of this info but, it should be good!
    Good luck!
  4. Jim Riggs

    Jim Riggs Karting

    Jan 25, 2004
    Thanks for the information:
    I calculated the following for the tire diameters in inches:

    Front: 25.43 original
    25.06 new

    Rear: 26.00 original
    26.63 new

    Using the formula : (Width in mm)×(Aspect ratio)×2/2550 + Rim diameter(inches) = tire diameter in inches

    I guess the question is “Will the new tires mess up the geometry of the suspension after alignment and adjustment and reduce the handling capabilities of my F-car or are they close enough to the original sizes?”

    Thanks, Jim

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