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.
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.
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
