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How to Read Speed Rating, Load Index & Service Descriptions
1.“225”: The numbers preceding the slash indicate the distance, in millimeters, across the widest point of the tire—called section width (225mm, or 8.85 inches, in this case)—when mounted on a wheel of specified width. Often listed before this number are letters that loosely signify the kind of duty for which the tire was designed: “P” stands for “p-metric” and is generally used on passenger cars, “LT” indicates light-truck duty, and “T” is for a temporary spare.
2. “60”: This two-digit number is the aspect ratio, or profile, of the sidewall. This tire’s sidewall height is 40 percent of the tire’s width, which equates to 99mm, or 3.9 inches. The lower the number, the shorter the sidewall. An exception is Michelin PAX tires, where this number signifies the overall diameter of the tire in millimeters.
3. “R”: This letter indicates radial tire construction; nearly all tires sold today are of this variety. Other constructions are “D” for bias-ply tires and “B” for belted. A preceding “Z” is simply a reference to an outdated and vague speed rating of more than 240 km/h, or 149 mph (the specific rating can be found in the service description).
4. “17”: This number indicates the diameter of the wheel on which the tire should be mounted, generally in inches. These are usually whole numbers but can also be half-inch increments, such as “16.5,” or in millimeters, as in “390.”
5. Load Rating: These numbers and letter together are called the service description. The numbers indicate the tire’s maximum “load” rating, or the amount of weight the tire can bear (“90” stands for 600 kilograms, or 1323 pounds), and the letter denotes the speed rating, or how fast the tires can safely rotate (“V” means 240 km/h, or 149 mph). The lowest rating typically found on passenger-car tires is “Q,” which means 99 mph. The highest, “Y,” is good for 186 mph, and when enclosed in parenthesis, as in “(93Y),” it means “in excess of” 186 mph. These values are determined by tire-testing machines in a lab, and the decoded load rating is also listed elsewhere on the tire.
6. UTQG: Is not a safety rating and not a guarantee that a tire will last for a prescribed number of miles. Under UTQG, manufacturers use three criteria to grade tires: treadwear, traction and temperature. The information is right where you need it when buying the tire: On the paper label affixed to the tread.
7. DOT Label: Every tire sold in the U.S. must have U.S. Department of Transportation (DOT) labeling. The first two characters indicate the factory of manufacture, and the next five or six are manufacturer-specific jargon (for tracking purposes, as in the case of a recall). The last four numbers give the date of production (the first two indicate which of 52 weeks, and the second two, the year). The European equivalent of the DOT code may also be present (it starts with an “e”), although fewer manufacturers are printing both on a tire’s sidewall (to prevent gray-market shipments when currency exchange rates fluctuate). If this string of numbers ends with “-S,” it means the tire complies with European noise regulations.
8. Mountain Snowflake: Unlike the M+S rating, this icon indicates that a tire has met a minimum performance requirement in snow testing.
9. Air Pressure: Maintaining correct tire inflation pressure helps optimize tire performance and fuel economy. Correct tire inflation pressure allows drivers to experience tire comfort, durability and performance designed to match the needs of their vehicles. Tire deflection (the tread and sidewall flexing where the tread comes into contact with the road) will remain as originally designed and excessive sidewall flexing and tread squirm will be avoided. Heat buildup will be managed and rolling resistance will be appropriate. Proper tire inflation pressure also stabilizes the tire’s structure, blending the tire’s responsiveness, traction and handling.