HOW TO “READ” A TIRE SIZE
People ask me all the time to explain the numbers and letters found on the
sidewalls of tires. Here’s a typical description commonly found on a sidewall
and an explanation of what it means:
P185/75R14
- “P” stands for the service description. In this case, it is a passenger car tire.
“LT” stands for Light Truck, and a “Z” rating means it’s a performance tire. - The number 185 is the width of the tire measured in millimeters (sidewall-to-
sidewall, inflated at the correct inflation level and mounted on the right rim
size). - The number 75 is the aspect ratio of the tire (the ratio of height to width). In
this case, the height of the tire is 75 percent of 185 millimeters or 138.75
millimeters. - The “R” means it is a radial tire. If the tire is a high-performance tire, there
will be a “Z” designation here. The size will read P185/75ZR14. - The number 14 is the measurement of the rim diameter.
If you want to be confident about choosing the best tires for your needs, learn
how to read the Uniform Tire Quality Grading System (UTQGS). The UTGQS
information is molded into the sidewalls of the tires. This system grades tires on
three factors: tread wear, traction, and temperature resistance.
Tread wear grade gives a comparative rating based on the wear rate of a tire
when tested under carefully controlled conditions. For example, a tire graded at
200 should have its useful tread twice as long as a tire graded at 100. Obviously,
tire life (in miles) varies depending on actual driving conditions. Variation in
driving habits, service applications, attention to proper maintenance (that is,
rotation, wheel alignment, maintaining air pressure), and road conditions all
contribute to tread wear.
Traction grade reflects the tire’s ability to stop on wet pavement under
controlled conditions on asphalt and concrete. The traction grades from highest
to lowest are: “AA,” “A,” “B,” and “C.” A tire graded “AA” may have relatively
better traction performance than a tire graded “A,” “B,” or “C,” based on
straight-ahead braking tests. The grades do not consider cornering or turning
traction performance of the tire.