How to tell what size rim you have

Another group of stamping on certain types of tires is the Uniform Tire Quality Grading or UTQG. This grading and stamping is required for passenger car tires (i.e. P-metric and Euro-metric) in the all season and summer categories. Dedicated winter tires, Light Truck (LT-Metric, Euro-Metric Commercial, Flotation) and Motorcycle tires are excluded from this requirement.

Quality grading is designed to make the tire purchase decision easier for you. Ideally, the system is intended to provide simple, comparative data so you can make an intelligent buying decision. However, the ratings are based upon test results achieved under special conditions. This means it’s possible to misinterpret the comparative data as it relates to your individual driving habits, conditions, etc. You should still rely on your service or tire professional for assistance. 

Quality grading designates the comparative performance levels of a tire based on government-specified tests but commissioned by the individual tire manufacturers. All tire manufacturers are required to grade regular and all-season passenger tires in three categories:

UTQG

1. Treadwear
2. Traction 
3. Temperature

Treadwear
The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test course for 6,000 miles (9,600 km). For example, a tire graded 150 would wear one and a half times as well on the government course as a tire graded 100. However actual tire performance depends on driving habits, road characteristics, service practices, and other factors that can influence the outcome. 

Traction Grades AA, A, B and C
The traction grades from highest to lowest are AA (the highest), A, B and C. They represent how well tires stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. C-rated tires will have the lowest traction performance. 

WARNING: THE TRACTION GRADE ASSIGNED IS BASED ON A WET BRAKING (STRAIGHT AHEAD) TRACTION TEST AND DOES NOT INCLUDE CORNERING (TURNING) TRACTION.

Temperature Grades A, B and C
The temperature grades A, B, and C represent the tire's resistance to the generation of heat and its ability to dissipate heat when tested under controlled conditions on a specified indoor laboratory test wheel. Sustained high temperature can cause the tire’s material to degenerate and reduce tire life, and excessive temperature can lead to sudden tire failure. The grade C corresponds to a performance level all passenger car tires must meet under the Federal Motor Vehicle Safety Standard No. 109. Grades A and B represent higher levels of performance on the laboratory test wheel than the minimum required by law. 

WARNING: THE TEMPERATURE GRADE IS ESTABLISHED FOR A TIRE THAT IS PROPERLY INFLATED AND NOT OVERLOADED. EXCESSIVE SPEED, UNDER INFLATION, OR EXCESSIVE LOADING, EITHER SEPARATELY OR IN COMBINATION, CAN CAUSE HEAT BUILDUP AND POSSIBLE TIRE FAILURE.

You can read automotive tires like a book if you know the language, and doing so will help you buy the best ones and get the most miles out of them. Our video above takes you through the entire secret world hiding in plain sight on the side of your tire, but here are a few highlights. 

Width, height and size

Perhaps the most commonly discussed piece of data about a tire is a number in the format "XXX/XXRXX," which describes a tire's size and shape in a somewhat arcane way. Using 235/55R18 as an example, 235 is the tread width in millimeters. The higher that first number, the wider the tire. The 55 is the height of the tire expressed as a percentage of the width we just saw. The lower this number, the shorter and more aggressively the tire sits on the wheel. The R means the tire is of radial construction (they all are) and the 18 is the diameter of wheel the tire fits, expressed in inches. This string is a real hodgepodge of numbers but it carries the essence of a tire's applicability.

Your tire's birthday

In smaller type, typically at the end of a line that begins with the letters DOT, you'll find a four-digit sequence like 3219. This reveals your tire's manufacturing date, expressed as the week of the year followed by the last two digits of that year. In this example "3219" means the tire was made in the 32nd week of 2019. This is important information because tires can run out of life long before they run out of tread: Heat, UV, ozone and the quality of the tire's materials can end a tire's safe usage life long before it fails the Lincoln penny tread test. The tricky part is knowing how old is too old.

The largest tire chain in the US, Discount Tire/America's Tire, has a policy that suggests you replace tires after six years, which is a commonly mentioned rule of thumb though many consumers see it as a ploy to sell them new tires. The company won't even service tires that are over 10 years old, other than to take them off your car, but that isn't as definitive as it sounds. "There is no such thing as a 'freshness date' for tires," says Richard Sherman, tire and accident reconstruction expert at Robson Forensic. "I've seen tires fail in one year or last about 20 years," he says, adding that you should look for sidewall cracking or unusual tread appearance as signals that your tires could be failing in ways that aren't revealed by simple tread depth.

If there is anything like a universal truth about tire age it would be that you should replace any tire with a three-digit date code: The industry hasn't used those codes since 1999.

Under pressure

We've all heard the exhortations to check and maintain our tire pressure for the best fuel economy, handling and safety. All of that is true, but when you grudgingly stoop down at the gas station with that nasty tire inflator, all you see on your tires is a "MAX PRESSURE" number. That is not the proper inflation pressure, merely the highest at which the tire won't disintegrate or come off the wheel. The pressure you should inflate your tire to is located in one of a few places according to federal rules: On a door edge, door post or glove box door or inside the trunk, or on the fuel-filler door and in the owner's manual. That doesn't narrow it down much, but once you know the number it's easy to remember. Just don't inflate to the MAX PRESSURE on the side of the tire. 

Can you tell rim size by tire size?