There’s a number printed on the sidewall of every tire that shows important information Knowing what the numbers, letters, and symbols mean is like having the Rosetta Stone of tire information, and is vital for tire maintenance and purchasing new tires.
Tire Width
Tire width is one of the most critical components of a proper fit. Width is almost always measured in millimeters and may be preceded by a letter. The three-digit number is the width of the tire at its widest point, rather than the actual width of the tread. A “P” ahead of the number signifies a Passenger car. An “LT” is for a Light-Duty vehicle, such as a pickup truck or large SUV. The absence of a letter may also signify a common passenger tire.
Sidewall Height
Moving left-to-right along the tire label, the next number is the sidewall height, but it’s not in millimeters. The sidewall is measured in aspect ratio, and the number is a percentage of the width. If your tire reads 255/55R18, the sidewall height is .55, or 55% of 255 (140 millimeters).
Tire Construction and Wheel Size Compatibility
Following the aspect ratio, there is a letter that comes followed by a number. The letter denotes the tire’s construction and is commonly “R” for “Radial.” A radial tire features fabric woven into the tire at a 90-degree angle to the tread. This gives it added strength and durability. Less common is a “B” for “Bias-ply.” These are outmoded tires and typically only used on classic or vintage cars. Some tires come with a “Z”, which corresponds to the speed rating and is the only time you’ll find the speed rating on this part of the label. The letter is followed immediately by a number. This is the size of the wheel to which the tire fits, measured in inches. A tire can only fit a wheel that matches the corresponding size of the inner circumference of the tire.
Tire Load Capacity
After the tire/wheel size, there is often a gap, followed by a set of numbers and letters. This is the tire’s load index, or how much weight the tire can carry. The letters and numbers correspond to a detailed chart with a wide variety of load indexes. For cars that will be used for towing or carrying heavy loads this is critical information.
Speed Rating
On the right end of the listing is the speed rating. This is the top operating speed at which the tire can operate. This speed should only be reached if the tire is properly inflated and maintained. When buying new tires, the speed rating of the new rubber should match or exceed the recommended rating in the owner’s manual.
Date-of-Manufacture
The date-of-manufacture label allows owners and service technicians to quickly identify the age of the tire. The letters D-O-T will be followed by batch- and plant-identifiers, which are needed to identify if a tire is part of a recall. These letters are followed by four numbers. The first two numbers are the week of the year, and the second two are the year itself. So 2618 is the 26th week of 2018.
Maximum Inflation Capacity
Elsewhere on the tire, is a “Max Press” or maximum inflation pressure. Tires should NOT be filled up to this number. Consult the owners manual to find recommended tire pressures. Recommended pressures can typically also be found on a label inside the driver’s side door frame.
Treadwear Uniform Tire Quality Grading
The Uniform Tire Quality Grading (UTQG) label shows a number of tolerances for the tire, including treadwear rating, temperature range, and the tire’s traction ability.
The treadwear rating test uses a reference tire, and the number (with a baseline of 100) indicates how much longer the tire can last, relative to the reference tire. A tire with a rating of 300 is expected to last three times as long.
However, the baseline tire is different for each tire manufacturer, so the treadwear rating may not be applicable from one tire brand to another. The best way to get a sense of a tire’s treadwear life is to reference the manufacturer’s warranty.
Temperature and Traction Uniform Tire Quality Grading
Tires have a recommended operating temperature. This can be found following the treadwear rating. There is a letter rating for traction grade and temperature grade. “AA” is the best, and “C” is the bare minimum for tires sold in the United States. High-performance tires typically have a higher operating temperature, which is why they are often called “summer tires.”
Other Markings
Though not found on every tire, there are other markings to be aware of on some tires. Some high-performance tires are designed for a specific side of the car and a specific direction. This can be noted by a sideways arrow. Another symbol looks like a cartoon snail. This denotes a run-flat tire, which has become increasingly popular, especially among some luxury brands.
Another symbol that may be found is a snowflake inside what looks like a mountain. This denotes a winter/snow tire. These are specifically designed for snow operations. Some all-season tires are marketed for their winter capability, but are not on the same level as these dedicated winter tires.
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