Decoding Tire Size — How to Read a Tire Sidewall

Tire sidewall can tell you much more than just their size. For example, if you know how to read tire’s sidewall you can learn where and when your tire was made, and in some cases eve if it complies with pass-by noise regulations in Europe!

In this article we’ll teach you to decode the information behind numbers on the sidewall.

1. “245”: Numbers that come before slash indicate tire’s section width in millimeters. Section width is a distance across the widest point of the tire. So in this case, section with of the tire is 245mm, or 9.65 inches). Often you can find letters that stand before this number. They indicate kind of duty the tire is designed for. For example, letter “P” means “p-metric”. It is the most common type used for passenger cars. “LT” stands for light truck. “T” means temporary spare. If you’re new to the off-road world, but already want to сalculate the diameter of tires you need, define the tread type, suitable for your style of off-roading, or simply need to get through the size designations shown on the sidewalls of your tires, our new technical article on Off-Road Tire Size Measurements will provide you with the helping hand. To make your decision process a lot easier, convenient chart of the most common off-road tire sizes
in inches and their metric equivalents is included. Read the full article at CARiD.

2. “40”: 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 98mm, or 3.86 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 A preceding “Z” is simply a reference to an outdated and vague speed rating of more than 240 km/h, or 149 mph (the specific rating can be found in the service description).

Decoding Tire Size

Decoding Tire Size

4. “18”: 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. SERVICE DESCRIPTION: 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 (“93” stands for 650 kilograms, or 1433 pounds), and the letter denotes the speed rating, or how fast the tires can safely rotate (“W” means 270 km/h, or 168 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. RED DOT: Here’s a long-held myth that can be put to rest: It doesn’t help in the balancing process to align the “heavy spot” of a tire—often indicated with a red dot—with a wheel’s valve stem when mounting. Aluminum wheels are now the norm, and the valve stem is no longer the de facto lightest point.

7. “200”: The tread-wear grade is a relative figure based on the rate of wear of a tire during a 7200-mile on-vehicle test compared with that of a reference tire. The higher the number, the longer it will likely last. “300” indicates that the tire should last three times longer than the Uniroyal reference tire, which scores 100.

8. “A”: A tire gets a seemingly uninformative traction grade (“AA,” “A,” “B,” or “C”) based on how much grip it generates in a straight-line test in which the tire is dragged at 40 mph across a wet surface without being allowed to rotate at all.

9. “A”: This letter indicates a tire’s ability to dissipate heat. As heat increases dramatically at high speed, this is, in effect, a second, less precise speed rating. “A” means the tire can withstand speeds over 115 mph, “B” is for between 100 and 115 mph, and “C” means between 85 to 100 mph.

10. “M+S”: This stands for “mud and snow” and simply means that the tire has more space between the treads, which should help to facilitate traction on soft surfaces.

11. “MOUNTAIN SNOWFLAKE”: Unlike the M+S rating, this icon indicates that a tire has met a minimum performance requirement in snow testing.

12. ORIGINAL EQUIPMENT (OE) MARKING: These letters—or a symbol—indicate that this is the automaker-specified version of a tire that came as a car’s original equipment. These tires can often be a very different blend of rubber compound compared with the off-the-shelf variety of the same tire, even though the tread pattern is identical. Examples of OE markings: General Motors—all have a TPC SPEC number; BMW—most have a five-pointed-star symbol; Mercedes—some are emblazoned with “M0”; Porsche—all have the letter “N” followed by a number, i.e., N1, N2, etc.

13. TIRE CONSTRUCTION MATERIALS: This is a required and self-explanatory list of the reinforcing materials and number of layers (in both the tread and the sidewall) that are molded into a tire’s rubber for reinforcement.

14. DOT LABEL: DOT stands for Department of Transportation. All tires sold in the U.S must have DOT label. So let’s learn how to read DOT labeling. The first two characters show the factory where tire was manufactured and the following five or six characters are manufacturer number that can be used for tracking purposes. This number is specific for the manufacturer. Four numbers that come last signify the date of production. The first two stand for a week, and the last two, for a year. Few manufacturers also add the European equivalent of the DOT code.

  1. RXCrider 7 years ago

    Pat and Tire Rack are both correct however most people ( until the advent of TPMS ) rarely even checked their pressures until it was too late. As for myself I check my pressures every 30 days , all my vehicles do not have TPMS ( thankfully )

  2. Patrick B. 7 years ago

    I notice one label on every tire that you guys neglected to include in this article and that is the recommended maximum tire inflation.
    My father used to swear that no matter what was printed on the sidewall, the maximum should always be 32 psi. (this having been told to him by the $8/hr tire jockey that mounted his tires).
    I have to disagree with that though and argue that the number printed on the tire should be used figuring the engineer that designed the tire would know better than the $8/hr grease monkey. And since many performance tires (as well as truck tires and such) recommend higher pressures on their sidewalls (Ive seen 42 psi and up to even 50 psi on several), plus the fact that its dangerous to run a tire under-inflated for too long as it heats the tire up which breaks down the rubber over time causing the potential for blowouts of the sidewalls.
    So Im curious as to which is right; 32 psi or whatever is printed on the sidewall…?

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