What Do the Numbers on Tires Really Mean?
Chances are, when the tread on your tires starts to show too much wear, you start looking for deals at nearby tire stores. Unfortunately, the set of tires on sale for a hundred bucks are almost never the tires that will fit your car or truck. Truth is most people don't spend a lot of time looking at the various numbers molded or imprinted on their vehicle's tire. But, just like the numbers on a spark plug, these provide important information and identify the characteristics and strengths required for different types of driving.
Let's get started with the tire nomeclature listed below:
- Three Digit Number - This number that appears before the forward slash is the tire width measured in millimeters at the widest part of the tire. NOTE: It is not the tread width. If the number is 245, then it is 245 millimeters wide.
- Two-Digit Number - The two-digit number that is displayed after the forward slash is a measure of the tire's height above the car's rim or wheel. The height of a tire is expressed as a percentage of the tire's width. For example, a 245/45 would be 45% of 245 millimeters or just over 110 millimeters tall.
- Two-Digit Number after the Letter - This number refers to the diameter of the wheel or rim that tire is mounted on. If it is a 245/45R17, then the opening in the tire will slide over a 17-inch wheel. The letter "R" stands for radial ply and the letter "B" stands for bias-ply tires. Bias-ply tires have become somewhat extinct for passenger vehicles.
- Second Three-Digit Number - The second three digit number is typically followed by a letter and represents a service description. You can look on your vehicle's placard or in the driver's manual to see what tire load your vehicle requires in pounds. The tire's load index number can be looked up in a table. For example a 98 load index is load rated for 1653 pounds. Multiply by 4 and your overall tire load capacity is 6,612 pounds for the vehicle, passengers and cargo.
- Speed Rating Letter - The letter after the load index identifies the speed rating for the tire. The letter R=106 mph and is used for heavy-duty light truck tires, whereas the letter H=130 mph is commonly used for sport sedans and coupes.
- DOT Code - The Department of Transportation Code numbers and letters identify batch runs and plant locations. These are used in case of a tire recall. The four numbers located in an oval represent the week and year that the tire was manufactured.
- Max Air Pressure - On most car and truck tires there is a listing for MAX LOAD and MAX PRESS (air pressure). NOTE: You should always check your owner's manual or vehicle's placard for the recommended air pressure. Inflating your tires to maximum pressure will cause a poor ride and uneven tire wear.
- Uniform Tire Quality Grading Number - This three-digit number represents the expected tire wear with 100 used as a baseline. So, a 300 UTQG would mean the tread wear is expected to be three times that of a baseline tire tread.
- Temperature Letter - Tires have the ability to create heat and dissipate heat and this is represented with "A" thru "C" (lowest quality allowed in United States).
- Traction Letter - This is a measure of the coefficient of friction or g-force under braking on a wet skid pad. It does not attempt to evaluate tire tread or the tire's ability to resist hydroplaning. "AA" is the best or highest rating and "C" is the lowest.
- Tire Symbols - If you see a symbol that looks like a snail, you have a run-flat tire. A popular sidewall mark of M+S stands for an all-season tire approved for mud and snow. However, if you operate your vehicle in heavy snow, look for a tire that also has a snowflake symbol.
Now that you know what the numbers, letters and symbols mean, it is important to ensure you install the right tires for you vehicle... PERIOD. So, stop looking at the tread pattern and start looking for your vehicle's numbers. You do have some flexibility in selecting tire tread for "on and off" road use, tire wear, grip and temperature. Moreover, if you're driving a high-performance sports car, you'll probably want to opt for a high speed rating as well as a new set of E3 performance spark plugs.