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Tire sidewall information

What does the tire sidewall information mean?

Understanding tire sidewall information

Some information on the tire sidewall tells you about the tire type, size, load index, and speed rating, while other information details maximum inflation pressure, manufacturing date, and wear/traction/temperature characteristics.

tire speed rating
What the terms in the above image mean

Tire type

P designates a P-Metric passenger constructed tire built to US standards.
LT designates a light truck tire. LT tires are not part of the P-Metric family, but they are also built to US standards.

ST (Special Trailer Service) designates the tire for use on a trailer. Trailer tires have stronger sidewall construction to handle lateral stress Special Trailer Service (ST) tire
The absence of a letter indicates a passenger constructed tire built to European standards

Tire width

The tire’s width in millimeters, measured from sidewall edge to sidewall edge. The larger the number, the wider the tire.

Aspect Ratio

The aspect ratio is the ratio of the tire’s height to width. In this case, the height of the tire is 55% of its width (225mm X 55%= 123mm height). Ratios of 70 or lower indicate a short sidewall for improved steering response and better overall handling on dry pavement.


Most newer style tires are a radial design and are marked with an “R”. Older style bias belted tires are marked with a “B”.

Wheel diameter

This represents the wheel or rim diameter in inches.

Load Index

The tire load number represents how much weight each tire can support.

tire load index chart

Tire load index chart

Speed Rating

The letter represents the maximum speed the tire is designed to be driven for extended periods of time. The speed rating is not required by law.

tire speed rating chart

U.S. DOT Tire Identification Number

This begins with the letters “DOT” and indicates that the tire meets all federal standards. The next two numbers or letters are the plant code where it was manufactured, and the last four numbers represent the week and year the tire was built. For example, 3120 means the tire was manufactured during the 31st week of 2020. The other numbers are marketing codes used at the manufacturer’s discretion. This information is used to contact consumers if a tire defect requires a recall.

Maximum Permissible Inflation Pressure

This number is the greatest amount of air pressure that should ever be put in the tire under normal driving conditions. If you are carrying the maximum payload rating for your vehicle, you can inflate the tires to the maximum pressure listed on the tires. It is commonly misunderstood that this is the recommended pressure. It’s not. The recommended pressure is listed on a label in the driver’s pillar area.

Uniform Tire Quality Grade (UTQG) Standards

The U.S. Department of Transportation (DOT) National Highway Traffic Safety Administration’s (NHTSA) Uniform Tire Quality Grade Standards (UTQG) are designed to help tire purchasers make comparisons when purchasing tires. UTOG testing is required for passenger tires, but not for light truck tires, winter/snow tires, temporary spare tires, trailer tires, tires under 12″ in diameter and other select tires.

The tests are conducted by an independent testing company


The UTQG Treadwear Grades are based on actual road use where a company’s test tire is run in a vehicle convoy along with standardized Course Monitoring Tires. The test is conducted on a 400-mile test loop for 7,200 miles. The tires and vehicle alignment are checked/reset and rotated every 800 miles. The Course Monitoring Tire is assigned a grade and the company’s test tire receives a grade indicating its relative treadwear compared to the test tire. For example, a Treadwear Grade of 100 would indicate that the company’s tire tread would last as long as the test tire. A grade of 200 would indicate the tread would last twice as long, 300 would indicate three times as long, etc.

Unfortunately, since the treadwear is calculated at only 7,200 miles of use, the treadwear rating don’t really reflect real wold wear. In other words, the tire manufacturers extrapolate the tire’s expected treadwear based on only 7,200 miles of actual use. The extrapolations can be easily manipulated by the tire company’s marketing department.

How to use treadwear ratings

Since each company is allowed to extrapolate treadwear from the USTOG test, you cannot compare treadwear ratings from one tire manufacturer to another since each company uses its own bias when making the extrapolations. However, you can compare treadwear life between tire models from the same company.

Traction Grades

Traction Grades are based on actual tests where the tire is skidded across the specified test surfaces. The test is performed by installing the test tires on the instrumented axle of a “skid trailer.” The skid trailer is pulled behind a truck at a constant 40 mph over wet asphalt and wet concrete test surfaces. The skid trailer’s brakes are momentarily locked and the axle sensors measure the tire’s coefficient of friction (braking g forces) as it slides. Since this test evaluates a sliding tire at a constant 40 mph, it places more emphasis on the tire’s tread compound and less emphasis on its tread design.

Traction grades are from AA for an asphalt g-force rating above 0.54 (concrete g-force rating above 0.38, “A” for asphalt g-force rating above 0.47 (concrete g-force rating above 0.35, “B” for asphalt g-force rating above 0.38 (concrete g-force rating above 0.26, and “C” for asphalt g-force rating less than 0..38 (concrete g-force rating less than 0.26.

Temperature Resistance

The Temperature Grade represents the extent to which heat is generated and/or dissipated by a tire. The grade is established by measuring a loaded tire’s ability to operate at high speeds without failure by running an inflated test tire against a large diameter high-speed laboratory test wheel.

An “A” temperature rated tire is run at speeds over 115-mph

A “B” temperature rated tire is run at speeds between 100-mph and 115-mph

A “C” temperature-rated tire is run at speeds between 85-mph and 100-mph and is the minimum required for sale in the U.S.

©, 2021 Rick Muscoplat

Posted on by Rick Muscoplat


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