Explanation of Tire Sizing and Rating

Tires are a very important component of a vehicle, since their quality has a significant effect on performance and safety. Replacing old, worn tires with quality new ones can make your vehicle feel like a completely different car. You have probably noticed that new tires have a series of letters and numbers noted along the outside edge, the meanings of which we will examine here.
The easiest way to understand the specifications abbreviated on tires is by using an example. So, assume a tire is marked with the following set of numbers and letters:

P225/60R16 89V

First, the letter “P” denotes that this particular tire is designed for use on a passenger car. This portion may have the letters “LT” instead, for light truck, or may contain no letters at all.
The set of three numbers, 225, 60 and 16 refer to the tire’s size. The first number, 225, represents the width of the tire measured in millimeters, commonly called the “section width.” The second number, 60 in this example, is called the “aspect ratio” and represents the height of the sidewall expressed as a percentage of the section width. So here, the sidewall height equals 60% of 225 mm, or 135 mm. The third number represents the diameter of the wheel in inches, so the tire in our example fits a sixteen inch wheel. The letter “R” after the sidewall size means that this particular tire has radial construction.

The next specification, 89, represents the load index, a measure of the weight that a tire is rated to support. The load index is an important rating because as you can imagine, the load index for an eighteen wheeler would need to be much higher than that of a passenger car. A standard load index chart lists the supported weight corresponding with each load index value, with a load index of 89 supporting up to 1,279 pounds at maximum air pressure. The passenger car using this tire would have a maximum load capacity of 5,116 pounds, calculated by multiplying 1,279 pounds x 4 tires.

The letter following the load index rating represents the speed that the tire is rated to reach and maintain. Speed rating is an industry standard that specifies a series of letters to represent different maximum speeds for which tires are rated. The letter “V” specifies that the tire is rated for speeds up to 149 miles per hour. Using the tire size and ratings noted in your vehicle owner’s manual is a good rule of thumb to ensure that the tires you purchase are properly equipped for your vehicle.

About the Author: Greg Chapman, of Greg Chapman Motors, is a leading used car dealer Austin has depended on since 1959. For more information please visit Greg Chapman Motors.