How to Read a Tire Sidewall

A Typical Tire Sidewall

Have you wondered what all those things on the side of a tire mean? Read this article and find out. Then compare your tires to the new information. See if your tires are really safe.

Things You'll Need

• Rag
• Flashlight

Instructions

1. 

   Discover the size: for instance 245/40R18 means -
   the numbers preceding the slash indicated the distance in millimeters across the widest point of the tire - called section width (245 m or 9.65 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, "LI" indicates light truck duty and "T" is for a temporary spare.

2. 

   After the slash: 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's PAX tires where this number signifies the overall diameter of the tire in millimeters.

3. After the slash continued: 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 A preceding "Z" is simply a reference to an outdated and vague speed rating of more than 240 km/h or 149 mph (the specific rating can be found in the service description).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. After the slash and still continuing onward: The final 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. 

   Next on the typical sidewall (after the size information) is the Service Description: These numbers and letters together are called the service description. The numbers indicate the tire's maximum "load" rating of the amount of weight the tire can bear ("93" stands for 650 kilograms or 14333 pounds). The following 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), 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. Next you should see the tire company's name. This is the manufacturer of the tire and is usually in larger easier to read letters. Some may even be in white letters.

7. Next comes the Uniform Tire Quality Grade - Part 1 of 3 is the Tread-Wear grade: 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 scored "100".

8. Uniform Tire Quality Grade - Traction: A tire gets a seemingly uninformative traction grade ("AA", "A", "B" or even "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. Part 3 of the Uniform Tire Quality Grade is Temperature: This letter indicates a tire's ability to dissipate heat. As heat increases dramatically at high speed, this is in effect a 2nd less precise speed rating. "A" means the tire can withstand speeds over 115 mph. "B" is between 100 and 115 mph and "C" means between 85 to 100 mph.

10. After the company's name you should see a "M+S" if for instance the tire your are looking happens to be a Mud and Snow tire. If so this simply means that the tire has more space between the treads, which should help to facilitate traction on soft surfaces. A little "Mountain Snowflake", unlike the M+S rating, is an icon that indicates that a tire has met a minimum performance requirement in snow testing.

11. After the "snowflake" icon, you should notice some more letters: These letters or a symbol indicate that this is the auto maker-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 include: General Motors - all have a TPC SPEC number; BMW - most have a five-pointed-star symbol; Mercedes-some are emblazoned with "MO"; Porsche - all have the letter "N" followed by a number, i.e., N1, N2, etc.

12. Tire construction materials: should be displayed above the Original equipment marking: 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.

13. The government gets involved next - 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 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.

14. Perhaps you noticed a Red Dot earlier on as you viewed the sidewall. 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 the wheel' s valve stem when mounting. Aluminum wheels are now the norm and the valve stem is no longer the de facto lightest point.

15. Now you have a much better idea of what all those codes mean. If you found this article helpful would you please let me know by rating it after you have read it entirely. In this way I will know how to better help others in the future. Thank You!

Tips & Warnings

• Information can always change.

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