How to Read a Tire Rating

Common information on a tire sidewall.

If you look at the tires on your vehicle, or are about to buy tires, you may wonder what all those number mean on the side of the tire. Wonder no more...

Things You'll Need

• Tire

Instructions

1. 

   When people refer to tire ratings, they may be talking about one of a number of "ratings" listed on a tire. The most common numbers people look at on a tire is its size rating.

   The first part of a size rating is a letter designating whether the tire is designed for a passenger (P) tire, a light truck (LT) tire, or a commercial (C) application. You may also see a temporary (T) designation on spare tires.

2. 

   After the letter designation, you will find a three digit number followed by a forward slash / and a two digit number. The three digit number indicates the width of the tire (in millimeters) from sidewall edge to sidewall edge. As such, a larger number indicates a wider tire.

   The two digit number is called the "aspect ratio." Basically, it tells you the height of the sidewall as a PERCENTAGE of the width of the tire. A common tire size is 205/75. This indicates that the tire is 205 millimeters wide with a sidewall height of about 154 millimeters (75% times 205).

3. 

   Following these two numbers will usually be the letter R which stands for radial construction. Radial tires have been the industry standard for over 20 years, but you may also see diagonal bias (D) or bias belted (B).

4. 

   The final number in the tire size tells what size tire wheel (or rim) the tire will fit onto. This number is measured in inches and indicates the diameter of the wheel. Common wheel sizes are 13, 14, 15, and 16 inches, although some custom rims can be larger.

5. 

   After the tire size, you may find two more tire ratings: a number and a letter. (The load rating is not required by law, so may not be on your particular tire.)

   The number indicates the load index, or the maximum weight each tire was designed to carry. Unfortunately, there is no easy conversion to look at the number and be able to calculate the weight. The figure for this step is a Load Index Chart to help decipher this number. The top graphic shows a tire with a load index of 82. This load index corresponds to a load of 1047 pounds (per tire).

6. 

   Following the load index is the speed index. This letter indicates the maximum sustained speed the tire was designed to safely travel. In a similar manner, there is no easy way to tell the speed by the letter, letters do not go in order, and a "higher" letter does not necessarily indicates a faster maximum speed. The top graphic shows a tire with a speed index of S. This speed index corresponds to speeds up to 112MPH (180KPH).

   Disclaimer for the common sense challenged: Please note that this rating relates only to tire speed capability based on the tire's limits, and is NOT a recommendation to exceed legally posted speed limits; always drive within the legal speed limits.

7. 

   Another rating (required by law on most passenger tires) is the UTQG, or Uniform Tire Quality Grading, which is made up of three components: treadwear, traction, and temperature.

   The treadwear rating indicates a general treadwear rate. Ideally, the higher the number, the longer the tread will last. Unfortunately, the testing performed for this number is to drive a vehicle 7,200 miles on a special track in Texas, then compare it to a "standardized" tire that has made the same trip. Knowing that most tires today will last at least 5 to 6 times that distance, it is difficult to accurately extrapolate information. In addition, since treadwear is affected by conditions not specifically tested for (like road composition, alignment, and driving habits), it cannot be correlated to actual miles of wear.

8. 

   The traction rating is determined by the ability to stop on wet concrete and asphalt in a straight line. THIS RATING DOES NOT TEST FOR CORNERING, HYDROPLANING, ACCELERATION, OR DRY CONDITIONS. Tires can be rated AA (best), A, B, or C. Most (75%) tires are rated A, while only a few (3%) achieve the best AA status.

9. The temperature rating indicates the tires resistance to generating heat. At fast speeds on a hot day, a tire may deteriorate causing a blow-out or tread separation. Tires can be rated A (best), B, or C with the majority (59%) rating B.

   Take note that for traction and temperature ratings, each letter has a large margin of variablilty. As such, a tire that is a low "A" would probably perform similarly to a tire that is a high "B."

10. 

    In the inner-rim of the tire will be some other information. For completeness of this article, the two main ratings of interest are the DOT number and the Max Tire Pressure.

    Ideally, any tire you use with have a DOT imprint indicating that the tire meets (or exceeds) all US Department of Transportation standards for tires. Following the letters DOT will usually be a few letters and/or numbers indicating the plant the tire was manufactured. After the plant identification will be a four digit number: the first two digits indicate the week the tire was made while the last two digits indicate the year the tire was made.

11. 

    The inner-rim will also indicate the maximum COLD tire pressure for the tire. Note that this is NOT the recommended pressure for the tire. You can find the recommended pressure for your tire on the tire placard. Depending on the car, you may find this information on the vehicle door edge, door post, glove box door, or fuel door. The recommended pressure is determined based on your vehicle model to account for handling, comfort, fuel mileage, and longevity of your tires.

Resources

• Safercar.gov
• www.tires-easy.com - the home of low tire prices for cars, 4x4's and vans