Tire Tech 101 – Reading Tire Sidewall Numbers and Letters

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.

Reading a tire's sidewall will give you loads of information about the tire's characteristics

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.

Street tires, such as this Continental ExtremeContact DW, are designed to be used on a variety of surfaces and through all sorts of weather conditions. They come with an almost endless combination of tread design, depth and rubber compound

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.

R-compound racing tires, such as the Nitto NT01, are designed to be used primarily in the dry only. They have a very small amount of tread (usually just 6/32") and can be used in the wet but their main advantages are increased heat resistance, softer rubber and a different construction

With a UTQG treadwear rating of just 80, the Michelin Pilot Sport Cup is the rare racing tire that is also a factory production car tire. Ok, so it's only found on heavy hitters like the Porsche 911 GT3 and BMW M3 CSL. It still counts

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.

When you're serious about all-out dry grip for auto-x or the track, you'll make the move up to non-treaded racing tires, such as Hoosiers or these Avon Tech-R tires. Many of these racing tires have twin grooves cut in them, to satisfy Dept of Transportation (DOT) rules. They may be loud, horrible in the wet and expensive but the only thing better are slicks

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.

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