UNDERSTANDING TIRE MARKINGS

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Understanding Tire Markings and Ratings

"Tire Type" defines the proper use of the tire. P means this is a passenger car tire. An LT in the Tire Size means that the tire fits a light truck.

"Tire Width" is the width of the tire measured in millimeters from sidewall to sidewall. The example shown above is 215 millimeters.

"Aspect Ratio" is the the ratio of the height of the tire's sidewall to its width. 65 means that the height is equal to 65% of the tire's width.

"Construction" tells you how the tire was put together. The "R" stands for radial, which means that the body ply cords, which are layers of fabric that make up the body of the tire, run radially across the tire from bead to bead. A "B" indicates the tire is of bias construction, meaning that the body ply cords run diagonally across the tire from bead to bead, with the ply layers alternating in direction to reinforce one another.

"Wheel Diameter" is the width of the wheel from one end to the other. The diameter of the wheel in the example above is 15 inches.

"Service Description" is the combination of the Load Index and Speed Rating described below. For example, the Service Description shown in the tire diagram above is "89H". This tire has a Load Index of 89 (rated for 1289 pounds or 580 kg) and a Speed Rating of "H" (up to 130 mph).

"Load Index" is a number corresponds to the maximum load in pounds that a tire can support when properly inflated. You will also find the maximum load in pounds and in kilograms molded elsewhere on the tire sidewall.

"Load Range" In addition to the “Load Index” the “Load Range” of a tire gives the strength and ability to hold air pressure, based on the sidewall ply of the tire, and is denoted by letters (B, C, D, E, etc.).The term “Load Range” replaces the term “Ply Rating” formerly used to rate tire strength. Most passenger car tires are rated Standard (STD) or Standard Load (SL). Standard Load tires will not have their load range branded on the sidewall. Extra Load or Reinforced tires will have “XL”  or “RF” branded after the tire size.

Load Range and Load Pressure for Passenger Tires

Since light truck tires are often available in multiple load ranges, the appropriate load range is identified immediately following the size's rim diameter in The Tire Rack's descriptions.

• LT-metric, LT-Flotation and LT-numeric tires will be branded with their load range ("Load Range E" or "LRE") or their ply rating ("10 Ply Rated") on their sidewalls and list their appropriate load range letter in their descriptions as LT245/75R-16E, 7.50R-15D or 31x10.50R-15C.
• ST-metric sized special trailer service tires will be branded with their load range ("Load Range D" or "LRD") on their sidewalls and list their appropriate load range letter in their descriptions as ST205/75R15 LRC.

Load Range, Ply Rating and Load Pressure for Light Truck Tires

"Speed Rating" is a number that corresponds to the maximum service speed for a tire. "H" means that the tire has a maximum service speed of 130 mph. Please note that this rating relates only to tire speed capability, and is NOT a recommendation to exceed legally posted speed limits; always drive within the legal speed limits. Speed ratings are certified maximum sustained speed designations assigned to passenger car radials and high performance tires. Because of the evolution of high-speed passenger car travel, it was necessary to establish a way to rate a tire's high-speed capability. In the U.S., these ratings are based on tire testing in laboratory conditions under simulated loads (European testing uses actual road testing). For a tire to be speed rated by the U.S. Government, it must meet certain minimum government standards for reaching and sustaining that specified speed. Domestically, high performance tires must be speed rated. The tire industry defines high performance tires as those with speed symbols of "S" or greater and aspect ratios of 70 or lower. Conventional passenger car radials need only meet the minimum Department of Transportation standard of 85 mph. Speed symbols may currently be marked on a tire in any of three ways: 205/60ZR15; 205/60ZR15 89W; or 205/60R15 89W.

"DOT" means the tire is compliant with all applicable safety standards established by the U.S. Department of Transportation (DOT). Adjacent to this is a tire identification or serial number; a combination of numbers and letters with up to 12 digits.

"UTQG" stands for Uniform Tire Quality Grading, a quality rating system developed by the Department of Transportation (DOT) that measures each tire for quality of treadwear, traction, and temperature.

Except for snow tires, the DOT requires the manufacturers to grade passenger car tires based on three performance factors: tread wear, traction, and temperature resistance. The UTQG rating of each Goodyear passenger car and light truck tire is listed in the Car/Light Truck Tire Catalog.

Tread Wear

• More Than 100 - Better
• 100 - Baseline
• Less Than 100 - Poorer

The tread wear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test track. A tire graded 200 would wear twice as long on the government test track as one graded 100. Your actual tire mileage depends upon the conditions of their use and may vary due to driving habits, service practices, differences in road characteristics and climate. Note: Tread wear grades are valid only for comparisons within a manufacturer's product line . They are not valid for comparisons between manufacturers.

Traction

• AA - Superior
• A - Best
• B - Intermediate
• C - Acceptable

Traction grades represent the tire's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. The Traction grade is based upon "straight ahead" braking tests; it does not indicate cornering ability.

Temperature

• A - Best.
• B - Intermediate
• C - Acceptable

The temperature grades represent the tire's resistance to the generation of heat when tested under controlled conditions on a specified indoor laboratory test wheel. Sustained high temperatures can cause the materials of the tire to degenerate and thus reduce tire life. Excessive temperatures can lead to tire failure. Federal law requires that all tires meet at least the minimal requirements of Grade C.

Directional

Directional tires can only spin in one direction. The direction of spin should be indicated by an arrow on the side of the tire.

Asymetrical

Asymetrical tires are designed to only be used front or rear, but not both. The tires will conform to the different properties of each axle for better performance.

Run-Flat Tires

Run-flat tires have been available as factory equipment on several high-end cars for a number of years, including the Chevrolet Corvette. These tires feature a special design that keeps them locked in place on the wheel and maintains drivability. Some run-flat designs simply add enough sidewall stiffness to low-profile tires to operate at conservative speeds without air. Newer designs incorporate a composite liner around the wheel inside the tire to add support if air pressure is lost. That means you can travel with zero air pressure for a considerable distance (as far as 50 miles at 55 mph), until you find a safe place to stop.

Run-flat tires protect your wheels from damage in case of a loss of tire air pressure. They also protect you from having to stop on the side of the road, which can be quite hazardous. Because it can be difficult to tell if a run-flat tire has lost air pressure, most are installed with a tire-monitoring system .

Run-flat tires are available from several manufacturers, including Goodyear, Dunlop, and Firestone. Michelin also is developing the PAX run-flat tire system, which requires a special wheel, as well.