How to Read Your Tire Size

If you’ve ever paid close attention to your tires, you may have noticed that there are a lot of confusing letters and numbers printed on the sidewall. These numbers and letters indicate the tire’s size, type and performance rating.

It might seem like all of this information is just for auto mechanics, but it’s something that each driver should know. Continue reading to learn how to measure your tire:

  • Tire type: The first letter on your tire stands for its type. A “P” indicates that it’s a tire designed for a passenger vehicle, while an “LT” means it’s for a light truck. Other letters include “T” for temporary tires, “ST” for trailer tires and “C” for commercial tires. Different tire types require different inflation levels; consult your owner’s manual to learn more.
  • Tire width: A tire’s width is measured from sidewall to sidewall. The first three-digit number after the letter is the tire’s width, measured in millimeters. So a tire that reads “P215” has a width of 215 millimeters.
  • Aspect ratio: The aspect ratio is the height of the tire’s cross section to its width. The larger the aspect ratio, the bigger the sidewall. This ratio is indicated on the sidewall by a two-digit number after a slash mark on the tire. Our tire mentioned above may read “P215/65”; this means the tire’s height is 65 percent of its width.
  • Construction: Following the two-digit number, you’ll find another letter that indicates how the tire was constructed. The most common letters are “R” for radial, “B” for bias-belted and “D” for diagonal bias. The construction type depends on the type of vehicle the tire is for.
  • Wheel diameter: The next thing to look for in how to read your tire is the wheel diameter, which tells you the size of the wheel that the tire is intended to fit. This is indicated by another two-digit number following the construction letter. Our “P215/65 R15” tire is made for a passenger vehicle with a wheel diameter of 15 inches.
  • Load index: The load index shows the tire’s weight-carrying capacity and is indicated by a two-digit number following the wheel diameter. However, it’s important to note that the number is just a representative symbol and does not numerically correspond to the actual carrying capacity. For example, a 95 doesn’t represent 95 pounds.
  • Speed rating: As the name implies, the tire’s speed rating is the maximum speed capability of the tire. The letter varies based on a few different factors, including the tire’s construction and size. There are many different letters that can be used. An example would be an “S,” indicating the tire cannot safely travel more than 112 miles per hour.

Want to learn more?

We’ve covered quite a bit in this post, but if you’d like to learn more about how to know your tire size or if you have any other questions about your vehicle, just call our pros at Pat Doll Automotive LLC. We’re happy to answer any questions you may have.