How To Size Your Tires

Tire Sidewall Markings

There are two types of alphanumeric codes molded into tire sidewalls that indicate tire dimensions and some key characteristics. The ISO metric tire code is generally used for passenger cars while the high-flotation code is used for truck tires.

Tire Sidewall Markings What all the letters and numbers on your tire sidewall mean

ISO Tire Code

(1) The International Organization for Standardization (ISO) metric tire code consists of a series of characters:

  • An optional letter (or letters) indicating vehicle tire type: "P" for passenger car, "LT" for light truck, and "ST" for special trailer.
  • A three-digit number, the tire width in millimeters from sidewall to sidewall.
  • A two- or three-digit number, the aspect ratio of the sidewall height as a percentage of the tire width. If larger than 200, it is the tire diameter in millimeters.
  • An optional letter indicating tire construction: "B" for bias belt with sidewalls and tread of same material, "D" for diagonal, and "R" for radial. If no letter, the tire is a cross-ply.
  • A one- or two-digit number for wheel diameter in inches.
  • A two- or three-digit number for load index. Some light-truck tires are for dual use in pairs next to each other. Separate load indices specify single and dual usage. Tires without such separate numbers are not suitable for dual use.
  • A single character or two-digit/letter combination for speed rating.

High-Flotation Code

(2) The high-flotation code for light truck tires starts with a two-digit number for tire diameter in inches followed by "x," a separator character, and a three- or four-digit number for the tire cross-section in inches. Then comes;

  • Tire construction indicators as in the ISO code, "B" for bias belt, "D" for diagonal, "R" for radial.
  • A two-digit number for wheel rim diameter in inches.
  • A two- or three-digit number for load index.
  • A one-character or two-digit/letter combination for speed rating.

There may be numerous other markings on typical tires displaying either code:

  • Arrows on directional tread designs showing which way the tire should rotate when moving forwards. It is important not to mount a clockwise tire on the left hand side of the vehicle or a counter-clockwise tire on the right.
  • "DOT" for the federal Department of Transportation specifying manufacturer, factory, mold, batch, and production date by two digits for the week of the year and two more for the year.
  • "M+S" or "M&S" for mud and snow indicating winter tires with self-cleaning tread and average traction in muddy or very snowy conditions and for low temperatures.
  • "M+T" or "M&T" for mud and terrain, tires to perform in mud or on terrain demanding additional traction.
  • "SFI" for side facing inward on asymmetric tires.
  • "SFO" for side facing outward on asymmetric tires.
  • "TL" for Tubeless, "TT" for use with an inner tube.

Tire Sidewalls: Example "P205/65R16"

Tire sidewalls show either of two types of tire sizes, ISO metric or high flotation. In metric tire sidewall codes the tire class is first. In this example, "P205/65R16," "P" indicates a passenger car. The code next indicates the tire width in millimeters (mm) sidewall to sidewall. In this example, the section width is 205 mm.

The "65" is the aspect ratio, the height of the sidewall as a percentage of the section width. In this example, the height is 65 percent of the section width of 205 mm or 133.25 mm. The "R" in this example indicates the type of tire construction, radial, and the "16" is for wheel diameter in inches.

Tire Sidewalls: Example "35x1250 R 17/E 92H DOT CC9L 1105"

In the high-flotation sidewall code example "35x1250 R 17/E," "35" indicates the tire diameter in inches. The "1250" indicates standard tire width in inches with no decimal point. Here, the tire is 12.5 inches wide. The "R" is for radial tire construction, and the "17" indicates wheel diameter in inches.

The last character, "E," indicates tire load carrying capacity. A "C" would indicate a 6-ply load carrying capacity. The tire is not actually 6-ply but contains one or two plies of such load-bearing strength. A "D" is an 8-ply rating, and an "E" is a 10-ply rating. With no final letter, the tire has a standard 4-ply rating.

The load index, "92," following the wheel diameter or load capacity indicates the maximum weight the tire can carry safely. Load indices range from 0 to 279 and for passenger car tires typically from 75 to 105. It is important to maintain the proper load index when replacing tires.

The speed rating, an "H," for example, after the "92" load index, is from the federal government for meeting minimum standards for reaching and sustaining specified speeds. In general, the higher the speed rating the better the vehicle handling.

The "DOT" indicates that the tire meets or exceeds the federal Department of Transportation tire safety standard. The first two letters, here "CC," following the "DOT" are codes to identify the tire manufacturer and the manufacturing plant. The third and fourth characters after "DOT" represent the tire size, and the final four digits represent the week and year in which the tire was manufactured, "1105 indicating manufacture during the eleventh week, sometime in March, of 2005.

Tire Sidewalls: Example "Treadwear·520 Traction·A Temperature·A"

The treadwear rating indicates tire durability but not tread life. Road surfaces, driving habits, and other variables affect actual tread life. Each manufacturer rates tread wear according to its own tests and not to any single industry or government standard.

Traction ratings measure tire ability to stop on a straight, wet surface under controlled conditions. It does not indicate cornering capability on wet surfaces nor traction on ice or snow. Traction grades are AA (the highest), A, B, and C.

The temperature rating measures tire resistance to heat generation under normal operating conditions at recommended inflation pressures. Temperature ratings are A, the highest and most resistant to heat generation, to C.

Tire Sidewalls: Example: "Max Load 635 kg [1400 lbs] at 300 kPa [44 psi]"

Maximum load limit, "635 kg [1400 lbs]" in the example, indicates tire maximum load-bearing capabilities at maximum inflation pressure indicated on the sidewall. The National Highway Traffic Safety Administration (NHTSA) sets maximum load limit standards.

Maximum air pressure, "300 kPa [44 psi]," indicates maximum tire operating inflation pressure, not the manufacturer's recommended inflation pressure, nor does it indicate the proper tire inflation pressure for the vehicle. This category is also based on NHTSA standards.

Some tire sidewalls indicate the direction of rotation, others indicate which side of the tire faces outward from the vehicle, and yet others may show both indications. It is important to mount tires according to such indications. Directional tires show arrows on the sidewall to indicate rotation moving forward. Asymmetrical tires show "Outside" on the side that should face outward from the vehicle.

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