By Ralph Kalal
You may think of it merely as the tire size, but it is more than that: numbers such as “P235/55R 17 98W” also specify the “aspect ratio,” “load index,” and “speed rating.” Other codes on the sidewall provide further information, including tire construction, age, and origin.
Taking tire size first, the “P” indicates that this is a “P-metric” tire size. Not all tires use this system. If there is no letter, just the numbers such as “235/55R 17,” it is referred to as a “metric” or “Euro-metric” size. Either way, the number is the width of the tire at its widest point, in millimeters. This is the “section width” of the tire.
The number after the slash is the “aspect ratio,” or “profile,” sometimes referred to as the tire’s “series.” Section width is a measurement. Aspect ratio is a percentage. It expresses the distance from the bottom of the tread to the wheel rim as a percentage of the tire’s width. So, a 235/55R 17 is 55% as tall as its section width and could be called a “55 series” tire. Any tire with an aspect ratio of 65 or less (i.e., a smaller number) is considered a “low-profile” tire.
The “R” stands for “radial.” The next number, in this case “17” is the rim diameter expressed in inches. Whether P-metric or metric, rim diameter is always given in inches.
The next two numbers and one letter are the “service description.” The numbers are the “load index” or “load rating,” which designates the maximum weight the tire can safely carry. The letter is the “speed rating,” which is the maximum safe speed for the tire. The load index and speed rating must be cross-referenced to charts giving the respective ranges to be decoded. Doing that, “98W” is a load index of 750 kg. (1693 lbs.) and a speed rating to 168 mph.
A tire with a speed rating of H or higher (i.e., V, Y, or W) and an aspect ratio of 65 or less is considered a “high performance” tire. A “Z rated” tire is speed rated for speeds above 149 mph. Z rated tires will show the “Z” immediately before the “R” in the tire size: 235/55 ZR 17. But the real speed rating is the W or Y in the service description, which gives the tire’s maximum speed capability. Currently, Y is the maximum speed rating: 300 kmh or 186 mph.
Some tires follow the size and service description with “M&S” or “M+S.” On some of those tires, the M&S is followed by a symbol in the outline of a mountain peak, in which there is an outline of a snowflake. M&S means it is a “winter tire,” a tire meeting standards set by the Rubber Manufacturer’s Association (RMA) for tires giving superior traction in sleet and snow. If M&S is followed by the mountain/snowflake symbol, the tire meets the RMA’s standards for “severe snow service,” a true “snow tire.”
The sidewall also says how the tire is constructed by specifying the number of plies in both the sidewall and the tread and the material used in each. So, “Plies: Sidewall 2 Polyester Tread 2 Polyester 2 Steel 2 Nylon” is a 6-ply tire: two polyester plies running from bead to bead, which also count as the two polyester plies under the tread; two steel belts around the circumference of the tire; and, two nylon belts on top of the steel belts.
Though winter tires are exempted, federal law requires other tires to carry grades for tests under the “Uniform Tire Quality Grading System” (UTQGS). There will be spot on the sidewall reading something like this: “Treadwear 300 Traction AA Temperature A.” Treadwear is measured from 600 (good) to 60 (bad); traction from AA, to A, B, and then C; and, temperature is A, B, C.
The UTQGS tests are federally mandated, but there is no federal standard for converting the results into the grades listed on the tire. Since the tire manufacturers do the testing and the grading, the scores don’t mean much unless you’re comparing two tires from the same tire manufacturer.
The tire also has a “DOT number:” the letters “DOT” molded into the sidewall followed by a series of numbers and letters, such as “DOT N9 EK F661 U205.” This is not a serial number and it is not a number unique to that tire,but it does tell you where and when the tire was made.
The “DOT” is a certification that the tire meets all applicable standards set by the federal Department of Transportation. The next letter/number—“N9” in this example—is a code for the specific manufacturing plant at which the tire was made, including plants in foreign countries. (The list is online at ftp://ftp.nhtsa.dot.gov/Manufacture. Unzip the “manufacturer” and then “new tire” files.)
The very last three numbers in the string (four, in some instances) are the week and the year in which the tire was made. “205” is the second week of 2005. The number “1006” would be the tenth week of 2006. For tires made before 2000, only the last digit is the year, and the first one or two are the week: “129” means the 12th week in 1999. The rest of the DOT number is comprised of control numbers used by the manufacturer that can identify tires in the event of a recall.
Many tires sold in the United States also display compliance with European tire standards set by a United Nations entity, the Economic Commission for Europe (ECE). A tire meeting those standards displays a symbol, the “E-Mark,” consisting of a box or circle with a the letter “E” and a number inside, such as “E3.” Following the symbol are numbers, sometimes followed by “s.” The letter/number designates the country in which the tire was first registered, usually the country in which the manufacturer is based. The following numbers refer to standards, many of which are equivalent to DOT standards. The “s” stands for “sound,” and means the tire passes a test for reduced tire noise.
There are also three codes designations that are unique to light truck tires: “LT” means it is designed as a light truck tire. “Max Load Dual” and Max Load Single” refer to the load capacity of the tire at a cold inflation pressure when used, respectively, as on a dual tire and single tire axle.