by Greg Wilson
With all the recent news reports about overloaded, underinflated tires contributing to tire failure, and a study by the Rubber Manufacturers Association that revealed most people don’t know where to find the manufacturer’s recommended tire pressure (in the vehicle’s owner’s manual, on the door jamb, or in the glove box), a brief overview of how to read a tire’s sidewall is a timely subject. Goodyear recently released this guide to deciphering all those abbreviations and symbols found on the side of a tire:
- P designates a tire primarily designed to fit passenger cars. Other letter possibilities are LT for light truck and T for temporary spare. Actually, the prefix letter is optional so it might not be included in all cases.
- 215, for example, the first number in the size designation, is the cross-section width (from sidewall to sidewall at its widest part). of this tire in millimeters.
- 65, the aspect ratio, is the percent of cross- section height (from the bead that holds the tire on the rim to the tread) to cross-section width For example, a 215-mm or 8.5-inch-wide tire with a 65 aspect ratio has a section height of 5.5 inches (8.5 x 0.65).
- R means radial tire construction. Other possible construction letterings are B for belted-bias construction and D for diagonal (bias) construction.
- 15 is the rim diameter in inches. The range of rim diameters on today’s passenger cars is 12 to 20 inches.
- M+S or any combination of the letters M and S signifies mud-and-snow certification, defined by tread patterns with angled slots and grooves that are a minimum 25 percent of the footprint. M+S tires are meant to provide improved starting, stopping and driving performance in the snow.
- The Rubber Manufacturers Association (RMA) and Rubber Association of Canada (RAC), which includes Goodyear, took the global industry lead in 1999 in designating winter tires that carry the mountain/snowflake symbol on the sidewall.
This severe snow service designation is a more stringent requirement than the M and S rating.
- Directional tires, such as Goodyear’s new Aquatred 3, increase the ability of the tread design to disperse water from under the tire for improved wet traction. An arrow on the sidewall, primarily added for the tire installer, shows the direction in which the tire should rotate.
- Although the load index numbers are too numerous to list here, the important thing to remember is that a higher load index number means a higher load capacity. A tire should never be replaced with another tire having a lower load index number.
The speed rating letters include S for speeds up to 180 kph; T, up to 190 kph; H, up to 210 kph; V, up to 240 kph; Z, 240 kph and more; W, up to 270 kph; and Y, up to 300 kph.
A speed rating indicates the maximum speed at which the tire may be operated, but it does not imply that the vehicle may be safely driven at that speed, particularly under adverse conditions where speed limits are mandated by law.
- Tire manufacturers are required to grade their passenger car tires for treadwear, traction and temperature resistance and assign Uniform Tire Quality Grading Labels according to regulations established by the U.S. National Highway Traffic Safety Administration.
Tire manufacturers, not the government, test tires and assign their own grades.
- The treadwear grade or number, according to the U.S. Department of Transportation, is for comparison purposes only and is not intended to be converted into anticipated tire mileage.
Since each manufacturer assigns its own number, a tire graded 220 by one maker might not deliver as much actual tread mileage as one graded 190 by another company.
- Temperature grades – A (the highest), B and C for tires that are properly inflated and loaded – represent a tire’s resistance to heat and its ability to dissipate heat when tested under controlled laboratory conditions.
- Traction grades – AA (the highest), A, B and C – represent a tire’s ability to stop on wet asphalt and concrete.
- The DOT symbol certifies compliance with Department of Transportation tire safety standards and is followed by a tire identification or serial number. For tires manufactured before the year 2000, the last three numbers identify the week and year of manufacture.
For example, 459 means the 45th week of 1999.
Beginning in 2000, the last four numbers identify the week and year of manufacture. For example, 3500 means the 35th week of 2000. Other characters in the serial number are the tire-maker’s coding for tire size, type and manufacturing plant.
- Maximum load limits and associated maximum cold inflation pressures are indicated in both metric and English units.
For normal operation, follow inflation pressure recommendations on the vehicle placard located on the driver’s side door post or in the glove box (and in some vehicle owner’s manuals), not the maximum inflation molded on the sidewall.