What Do All Those Words & Numbers on My Sidewalls Mean?

Though some of it looks like a secret code, all the information you find on the sidewalls of
your tires has a purpose and a meaning. We’ll try to crack some of the codes for you.

Tire Size

DOT Number

Ply Rating / Load range: The following chart shows the relationship between load range and ply rating (PR). These designations provide a relative indication of maximum inflation pressure and the corresponding maximum load for which a tire is designed. Also sometimes considered an indicator of relative casing strength.   Ply Rating Load Range 2 A 4 B 6 C 8 D 10 E 12 F 14 G 16 H 18 I 20 L 22 M 24 N

Maximum Load Ratings: Maximum loads depend on inflation pressure and on whether the axle has a single tire and wheel assembly, or duals (two bolted together). .Dual load ratings are somewhat lower than single ratings to compensate for typical mismatching of inflation pressure, as well as momentary overloading if the tires encounter an uneven road surface. Do not exceed the maximum load or maximum inflation pressures shown on the tire sidewall. Lower inflation pressures may be used, but only with lighter loads. More detail is provided in manufacturer’s load and inflation tables. "Cold" means that the tire has been run less than one mile during the past 4 or more hours. On an extended trip in hot weather, heat can cause tire pressures to rise by as much as 15 to 20 percent.

Maximum Load Ratings: In this example, a single steel body ply runs from bead to bead. The sidewall, therefore, has one steel ply. Four steel belts are added between the body ply and the tread to make a total of five steel plies in the tread area.

 

Why are so many of the dimensions referred to as “nominal”? Exact tire and wheel dimensions may vary slightly, and may depend to some extent on inflation pressure, loads, and other factors. "Nominal" dimensions correspond to general industry standards, and usually are close to actual dimensions. What’s the use of all these codes? Besides providing useful information about sizes and correct usage, these codes have additional functions. The manufacturing date can be very important in determining whether a tire is in warranty, or in deciding whether or not to retread. And, for proper service, all load, inflation and safety warnings should be observed carefully by everyone who works with tires. Aspect Ratio = (Nominal section height / Nominal section width) X 100 Typical aspect ratios for low profile tires might be 80, 75, 70, 65, even 60. Tall profile tires are considered to have an aspect ratio of 90 or 100. Tires with a given aspect ratio are sometimes referred to as "90-series," "75-series," etc.