Decoding Tire-Size Code

by Don Elliott on July 28, 2011

Understanding the markings on the side of an automobile tire provides important information when selecting tires for your vehicle.  Most tire sizes are given using the ISO Metric sizing system. Additional information from the Uniform Tire Quality Grade (UTQG) rating system includes traction, tread wear, and temperature resistance guidance.

Many tire manufacturers have added coding descriptions to their websites. While the information on the meaning of the codes is readily available, understanding how to interpret the code information is not so simple.

The information on the inner sidewall contains information that is not on the outer sidewall. The code has been standardized, but the location of the data has not. Some of the dimensions are metric and others are U.S. customary system.

The P-Metric sizing system for passenger cars is displayed on both sides of the tire like this:             P185/70R14 85S M+S.

            P – Indicates this is a passenger tire. LT would be a light truck tire.

            185 – Width across the tread in millimeters. To convert to inches, divide by 25.4.

            70 – Aspect ratio (section height divided by section width) expressed as a percentage.

            R – Radial tire construction.

            14 – Rim diameter in inches.

            85 – Load index or the maximum load each tire can carry, in this case 1,135 pounds.

            S – Speed rating. S indicates the maximum speed the tire can carry a load that corresponds to its load index.

            M+S – Mud and snow tire for traction in inclement weather. There should be a snowflake or mountain symbol accompanying this tread identity.

Other information that is listed on most tires includes:

  • Tire ply composition and materials used – describes the layers of rubber coated fabric or wire that make up the tire casing
  • Tread wear Rating- Measure of tread durability against an industry standard of 100
  • Traction – Measure of a tire’s ability to stop on wet asphalt and concrete. Grade A performed well on both surfaces. Grade B performed well on at least one of the surfaces. Grade C performed poorly on one or more surfaces.
  • Temperature Resistance – Measure of the tire’s resistance to heat. Grade A indicates that the tire withstood a half-hour run at 115 MPH. Grade B indicates that the tire passed at 100 MPH but not at 115 MPH. Grade C is the minimum performance level at less than 100 MPH.
  • U. S. DOT tire identification number – The letters DOT followed by code identifying the manufacturing location, tire size, manufacturer’s code, and date of production. Tire dealers must keep records of the DOT branding along with the tire buyer’s name and address according to federal law.
  • Maximum load rating
  • Maximum permissible inflation pressure
  • Tire and manufacturer names

Tire shops and auto repair shops make each side of the tire different to provide more options for tire buyers. One side of the tire might have raised white letters, while the other side could have raised black letters. White walls are only on one side of the tire. The other side will have black walls. To change the look of your used car, have your tire auto repair shop flip the tires on the rims at your next tire rotation.

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