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How to ‘Read’ a Tire’s Sidewall

Published Apr 6th 2007, 3:39pm by Jody DeVere in Featured Articles

Mary_small By Mary Aichlmayr, Managing Editor, Tire Review
Take a close look at the sidewalls of the tires you’re using right now on your vehicle. You’ll see several letters and numbers, which might appear random. Nothing could be further from the truth.

Regardless of manufacturer or brand, every tire sold today is required to be stamped with certain vital information.

Everything you need to know about a tire is stamped right into the rubber. So, knowing how to ‘read’ a tire’s sidewall is a great skill that comes in handy when it’s time to shop for tires.

So, here’s how to do it!

A tire’s size is written in this format: P215/65R15 89H.

The “P” on this particular tire means it is designed for use on passenger cars. If you see an “LT” before the numbers, the tire is a “light truck” tire, designed for pickups, SUVs and other small trucks.

Set_of_tires The number 215 represents the width of the tire, measured in millimeters, from sidewall to sidewall. In our example, the tire’s width is 215 millimeters.

The next number, 65, is known as the tire’s “aspect ratio.” A tire’s aspect ratio is defined as the height of the tire’s cross-section to its width. The 65 in the example above means that the height is equal to 65% of the tire’s width. When people talk about “low-profile tires,” they are talking about tires with low aspect ratios. You’ll often see these tires with their very thin sidewalls on sports cars and other high-performance vehicles.

The “R” refers to “radial,” the tire’s construction. In radial tires, layers of rubber, steel and nylon run radially across the tire. The alternative, bias construction, means that the layers run diagonally across the tire.

The number 15 in the example represents the wheel diameter, measured in inches, for which this tire is intended. Wheel diameter is the width of the wheel from one end to the other.

Lastly, the number 89 represents the load index of the tire – the maximum load in pounds that the tire can support when it’s properly inflated. The “H” after the 89 indicates the tire’s speed rating – the maximum speed that the tire can handle.

An “H” speed rating means that a tire has a maximum service speed of 130 mph. It is generally recommended that you replace a speed-rated tire with a tire having an equivalent or greater speed rating.

Here is an overview of maximum speeds, in miles per hour, for particular speed ratings:

Q: 99
S: 112
T: 118
U: 124
H: 130
V: Above 130
Z: Above 149

On passenger, performance and light truck tires, you’ll see another group of letters and numbers stamped on the tire’s sidewall, about 180 degrees from the size information. This is the tire’s UTQG, which stands for Uniform Tire Quality Grade. Knowing what UTQG is and how to interpret it can help you when shopping for tires.

The Department of Transportation (DOT) requires all tire manufacturers to grade passenger tires on three performance factors: tread wear, traction and temperature resistance. All testing is performed by each tiremaker using prescribed test tires and procedures.

The benefit to you, the consumer? You can easily compare the performance of two or more tire models and brands without ever driving on them.

Tires Here’s an example of a UTQG rating: 400 AB.

In this example, 400 is the tire’s tread wear rating, “A” is its traction rating, and “B” is its temperature-resistance rating.

Tested against an industry standard index of 100, the tread wear rating indicates how well the tread lasts compared with the reference standard. For example, a 200 tread wear rating means the tread wears twice as well as the standard, while a 400 rating (as in our example above) means the tire wears four times as well as the standard.

The UTQG traction measurement notes a tire’s ability to stop on wet test surfaces of asphalt and concrete under controlled conditions. The rating is based on a low-speed (40 mph) wet braking test and generally indicates the traction capability of the tread compound in straight-ahead braking.
Grade A means the tire performed well on both surfaces (as in our example); grade B means the tire performed well on at least one of the surfaces; and grade C means the tire performed poorly on one or both of the surfaces.

The UTQG test standards also measure a tire’s resistance to heat and its ability to dissipate heat when tested under controlled conditions on a specified indoor laboratory test wheel. The UTQG temperature-resistance test is conducted under predetermined standards for inflation and loading. Grade A is the maximum performance level, which indicates the tire withstood a 30-minute run at 115 mph without failing. Grade B indicates the tire passed 100 mph, but not 115 mph, and grade C is the minimum performance level, which indicates the tire failed to complete a half an hour at 100 mph. Federal law requires that all tires meet at least the minimal requirements of grade C.

It’s true that these ratings can be useful. But, always keep in mind that UTQG ratings generally do not represent a tire’s actual performance in real-world driving conditions. In the real world, driving habits, road and weather conditions, inflation pressure maintenance and other factors will impact the actual mileage that a tire delivers. And, the traction grades given in UTQG ratings denote only straight-ahead, wet braking on concrete and asphalt. They do not measure turning or cornering traction or hydroplaning resistance. Plus, speed, loading and inflation can significantly impact a tire’s resistance to heat, which in turn, can reduce a tire's durability and service life. In short, use UTQG as a general guide, not a hard-and-fast rule.

So, the next time you shop for tires, you’ll know exactly how to ‘read’ them. And, you’ll be a more informed, more educated tire buyer.

by Mary Aichlmayr, Managing Editor, Tire Review Magazine

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