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Tire Pressure Numbering System

The numbers used to identify passenger tires contain a lot of information buried in that long string of codes that appear on every tire rim. Here is a brief explanation. These codes apply to passenger car tires, but not to snow tires.

The traction index can be AA, A, B or C. This gives a relative indication of the tire's stopping ability on wet pavement.

The temperature index follows and can be A, B, or C. This is a relative indication of how well the tire stands up under various temperature conditions.

The tread wear index rates how well the tire resists to wear under "normal" conditions. A reference value is 100. Thus with in the same manufacturer's lot of tires, a rating of 300 suggests the tire should last three times as on rated 100.

The size code follows and shows several aspects of the tire size.

The first letter or letters shows the vehicle type the car is intended for.
  • P denotes passenger car.
  • LT denotes light truck.
Unfortunately this letter code is not always used.
The next several digits show the width of the tire in millimeters.
Following the slash are (usually) two digits that are a comparison of tire height to width.
A maximum speed rating sometimes appears here.
  • Q denotes a maximum of 100 mph.
  • U denotes a maximum of 124 mph.
  • Z denotes a maximum of 149 or more mph.
    The next field is construction type.
    • R denotes a radial tire.
    The next field - which is optional - identifies the factory where the tire was made.
    The last four digits denote the week and year the tire was made.
    • The first two digits show show the week, with the first week of the year being a one.
    • The last two digits show the last digits of the year.

    For example -

    P205/70R15 95S
    This has the following interpretation -
    • No traction index.
    • No temperature index.
    • No tread wear index.
    • P - for a passenger car
    • No maximum speed index.
    • The width of the tires is 205 millimeters. (25.4 millimeters = one inch).
    • R - The tire is of radial construction.
    • No factory identification is supplied.
    • The tire was made during the 15th week of 1995.
    • S - ?

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