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Thread: Tire size

  1. #1

    Default Tire size

    My 1978 Camaro has p205/75/14 craiger ss rusted beyond repair. Doesn't look like a lot of selection in 14'' tires if I bought new 14'' rims.
    Does anyone know how to calculate a 15'' rim and keep the original tire diameter.

  2. #2
    Join Date
    Nov 2005
    Posts
    12,678

    Default Ask...

    Quote Originally Posted by JMTC View Post
    My 1978 Camaro has p205/75/14 craiger ss rusted beyond repair. Doesn't look like a lot of selection in 14'' tires if I bought new 14'' rims.
    Does anyone know how to calculate a 15'' rim and keep the original tire diameter.
    If mine, I would talk to a local tire dealer. Tell them your size as you told us and ask what the same (height) would be in a 15".

    My broth/law just did this last fall on a new car with 18". He likes putting 4 snow tires on separate rims for the winter. He went (locally here) Town Fair Tire and per tire AND rim would be close to $400. Guy there suggested he go with a 17" rim and tire to equal the same size as the 18". He got the tires he wanted on their 17" mag wheel for $200 each out the door.

    They have all sorts of cross reference charts. Let us know.

    Henry

  3. #3
    Join Date
    Dec 2015
    Posts
    326

    Default Tire sizing

    On a 15" rim you would simply want a p205/70/15 tire. This would make your tires height .2" taller, side wall -.4 smaller and tire circumference .62" longer
    P= passenger
    205= side to side tire width in mm or (8.07 inches)
    70= Aspect ratio, in this instance the side wall height is 70% as wide as the tire width or 205x70=143mm(5.64" inches)
    15= rim size
    Total tire height would be 15+5.64+5.64=26.28" inches
    to convert mm to inches divide mm by 25.4=1 inche
    Example above P205(tire width 8.07 inches)/ 70 (aspect ratio of tire width to sidewall 5.64" sidewall)/15 rim width
    Last edited by Ronf; 08-13-2019 at 10:11 PM.

  4. #4
    Join Date
    Dec 2015
    Posts
    326

    Default After thought

    This formula allows you to calculate any tire size when doing wheel fitment. Just remember that rim width comes into play when speaking of tire width.,Tire width is calculated on the flat part of the tire as the car sits on the ground and the tire bulges from weight. This formula is based on 7" rim width. Using the same tire on an 8" width rim will expand the tire width about an extra .250" inch. In otherwords, for every 1" rim width using the same size tire you have to add .250" for tire fitment.
    Hope this helps
    Last edited by Ronf; 08-13-2019 at 10:22 PM.

  5. #5

    Default

    p205/70/15 ,from original p205/75/14 will give the same diameter. got it thanks. very helpful info.
    Jim
    Last edited by JMTC; 08-14-2019 at 10:38 AM.

  6. #6
    Join Date
    Nov 2013
    Location
    olympia,wa
    Posts
    797

    Default

    it's crazy hard to find the right size tires for some of our projects these days. remember when 13 inch tires were the cheapest thing on the market? try finding them now... one of my cars is supposed to have michelin xas 13's and the mold in the proper width has been lost. 13 inch tire selection is awful.
    b marler

  7. #7

    Default

    Yes in 14'' there is very little selection. I want very narrow tread so limestone rocks don't get stuck in tire and fly out at highway speed and chip the rockers.
    I bet you would pay a premium price for the smaller diameter tire now also.

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