Tire Tech – How To Read A Tire Sidewall
Tires are simple to understand, right? They’re all black, round and made of rubber. As long as they fit around the wheel, they should be good. Not really. As an OR reader, you wouldn’t go into the local Pep Boys and sling four of the best Chinese remanufactured specials (on sale for $80! – Ed.) on to your car. You want the best for your dollars and learning how to read a tire is the first step.
Printed on the face of every tire’s side, called the sidewall, is a series of numbers and letters. With these, you can figure out the tire’s general traction capabilities, age and size. Combine these specs with the price you’re willing to pay and you should be able to figure out the right tire for you.
Tire Size
The most important thing to pay attention to is tire size, printed in a format that will read something like “225/45R17”. Some tires will also begin with a letter, such as “P225/45R17”, where the “P” means that the tire is meant for passenger vehicles. Other letters include “LT” (light truck) and “T” (temporary spare).
The first number is the width of the tire’s face from sidewall to sidewall, measured in mm. The above example is 225mm across the face. The second number is the aspect ratio, also commonly referred to as the profile. This number is the height of the sidewall and is read as a percentage of the tire width. In our example, the sidewall height is 45% of 225mm, 101.25mm. The letter “R” means that the tire is a radial tire, the most common currently in production. It’s unlikely that you’ll run into any other letter in that spot. The third number is the wheel diameter.
Tire sizing is important because it directly affects tire fitment (i.e. rubbing), gearing and your speedometer. Your car’s speedometer is calibrated to the factory tire size and the tire’s circumference, i.e. distance around its face. Since tires are your car’s only connection to the ground, tire circumference is used as a function of rotation per time to figure out vehicle speed.
By using too large or too small of a tire, your speedometer will think you’re going a different speed than you really are. You can also use this to your advantage during autocross runs or at the track by using a thinner tire for a smaller overall diameter, shortening the gear ratio.
UTQG
UTQG, which stands for Uniform Tire Quality Grades, is a general system for judging overall tire characteristics. The ratings are broken down into three parts: treadwear, traction and temperature.
Treadwear
The UTQG treadwear rating ranks overall wear rate and softness. In general, the lower the number the higher the dry grip of the tire. Commonly found passenger car tires rank in the low 300s, ultra high-performance street tires in the low 200s to mid/high 100s and racing tires are usually 100 and below.
Traction
UTQG traction rates a tire’s braking grip over a wet surface under a controlled National Highway Traffic Safety Administration (NHTSA) test. Grades given are AA, A, B and C, with AA being the highest. This grade is only partially relevant for OR readers since the NHTSA doesn’t test for dry grip or cornering on a wet skidpad.
Temperature
UTQG temperature grades include A, B and C, with A being the highest. Grades are tested and given out by running a tire against a roller in a controlled test. This test is important because high-speed operation causes heat to build up in a tire. A higher temperature resistance will survive high-speed operation for longer periods of time.
Load/Speed
A tire’s load and speed rating is usually given in a format similar to “95V” and will follow the tire size, read completely as something similar to “225/45R17 95V”. The numbers are the tire’s load rating and matches to a general ability to carry weight. The letter is the speed rating and different letters (below) are used to designate a tire’s maximum speed rating.
| L | 75 mph |
| M | 81 mph |
| N | 87 mph |
| P | 93 mph |
| Q | 99 mph |
| R | 106 mph |
| S | 112 mph |
| T | 118 mph |
| U | 124 mph |
| H | 130 mph |
| V | 149 mph |
| W | 168 mph |
| Y | 186 mph |
| Z | 149 mph+ |
