B arry's T ire T ech

This is a series of articles on the technical aspects of tires, their care and usage.

My primary purpose in these articles is to help people understand tires and thereby reduce the risks we all face every day.

..........and since tires is just about the only thing I know about..........

Please drop me a note if you have a topic you want to see:

Barry@BarrysTireTech.com

Tire Sizing

One of the fundamentals about tires is sizing. There is a lot of confusion concerning tire sizing and this is going to explain it in general terms and basically how it works.

As a general rule, tire sizing describes the physical dimensions of a tire - but that is only part of the story. It also includes the way in which the load carrying capacity is calculated.

The current system for passenger car tires is like this:


P205/60R16

    where:

  • P = Passenger car

  • 205 = the width of the tire at its maximum point in millimeters (Section Width)

  • 60 = the ratio of the section height to the section width (in this case 60%)

  • R = Radial

  • 16 = the rim diameter in inches

  • I think it is obvious that given the above, the overall tire diameter is now defined.

    What is not so obvious is that this form also defines the load carrying characteristics - but it does - and that relationship takes the form of a table (commonly called the load table) where the maximum load carrying capacity is listed by inflation pressure.

    Barry's Tire Tech - Load Tables

But all other tire sizing systems have a simliar set up. They are encoded so that the width, the overall diameter, the rim diameter, and the load table are defined, thusly:


    7.35-14

where (see the table to the right from the 1970 TRA Yearbook - which you should click on and print out):

  • 7.35 is a code that means:

    • 7.30" section width (on a 5JJ rim)

    • It is a "Super Low Section" tire (which has an 82% aspect ratio)

  • "-" means it is a bias type tire

  • 14 = the rim diameter in inches

and all that would then define the overall diameter - which does not need to be calculated because the overall diameter is already specified in the table.

  • Note: If you actually do the calculation, you'll find the tire actually has an 81.9% aspect ratio.

But this form also encodes the tire load characteristics - that is, the relationship between inflation pressure and the maximum load at that pressure. It is shown on the page to the left. BTW, both that page and the one above are from the 1970 Tire and Rim Association Yearbook.

I would suggest clicking on the image to the left and printing out the page.

So that you get the idea, here are some other old tire sizes:

  • Super Ballon (95% aspect ratio)
    • 5.60-14
    • 5.90-14
    • 5.90-15
    • 6.40-14
    • 6.40-15
    • 6.70-15
    • 7.10-15

      I think it it obvious that the form is something that ends in .X0, where the "X" is a number other than a zero (0) or a five (5) - pronounced like "ten", "ninety", or "forty".<.p>
  • Low Section Height (88% aspect ratio)
    • 6.00-14
    • 6.50-14
    • 7.00-14
    • 7.00-15
    • 7.50-14
    • 7.50-15

      etc. of the form that ends in either .50 (pronounced "fifty") or .00 (pronounced "hundred")

  • >
  • Super Low Section (82% aspect ratio)
    • 6.45-14
    • 6.95-14
    • 6.95-15
    • 7.15-15
    • 7.35-14
    • 7.35-15

      etc. of the form .X5, where "X" is a number, and pronounced like "thirty five", "forty five", etc.

  • Millimetric (also 82% aspect ratio)
    • 165-13
    • 165-14
    • 175-13
    • 175-14
    • 175-15
    • 185-14

      etc., but in millimeters ending in "5"
  • But there are other ways to do tire sizing:


    30X10.50R15LT

      where:

    • 30 = the overall diameter in inches

    • "X" is a separator (and pronounced "by")

    • 10.50 = the Section width in inches

    • R = Radial

    • 15 = rim diameter in inches

    • LT = Light Truck

    This form is called "Flotation" - and it has its own set of load tables.

    I hope it is obvious that to define the physical dimensions of a tire, I only need 3 dimensions:

    • the rim diameter

    • and 2 of the following:

      • Overall Diameter

      • Aspect Ratio

      • Section Width

    I hope from the previous examples you can see that I can use also ISO (metric) units or English units - although rim diameters have historically been in inches, which continues to this day.

    - AND -

    the form the size takes also defines the load table to be used.

    - PLUS -

    There are usually separators: "-", "/", "X", and letters, but usually only the letters have meaning.

    Tire sizing has been an evolutionary process and as time has gone on, some lessons have ben learned.

    First is that you may think that "6.95", "30", or "205", are actual dimensions - and they are NOT!

    They are a code and the actual dimensions will be close, but not exactly that.

    In order to deal with that - and to simplify things, the Tire and Rim association came up with a sizing system called "AlphaNumeric", which had the form of:

      GR70-15

      where:

      • G = a reference to the load table

      • R = Radial (which would be omitted if the tire was not a radial)

      • 70 = aspect ratio

      • 15 = our old friend, rim diameter

    What was great about this was that the letters always stood for the load carrying capacity, so it was difficult to go wrong by undersizing.

    The bad news was that you needed a table to figure out the physical dimensions.

    But this sizing system started about the same time that radials started being used. Since radials used the same sizing sytem that people were used to seeing, albeit with metric units, the alphanumeric system was dicarded.

    If you deal with tires other than the current sizing sytem for passenger car tires, you will come across other types of sizing systems, but they will all have something in common:

    • They will all have encoded a load table
    • The size will define the physical dimensions in the way the size is stated.



    Barry's Tire Tech - Main Page

    Barry's Tire Tech - Load Tables

    Barry's Tire Tech - Dimension Tables