Each section of small print on a tire's
sidewall means something:
Tire Type
The P designates that the
tire is a passenger vehicle tire. Some other
designations are LT for
light truck, and T for
temporary, or spare tires.
Tire Width
The 235 is the width of the
tire in millimeters (mm), measured from
sidewall to sidewall. Since this measure is
affected by the width of the rim, the
measurement is for the tire when it is on
its intended rim size.
Aspect Ratio
This number tells you the height of the
tire, from the bead to the top of the tread.
This is described as a percentage of the
tire width. In our example, the aspect ratio
is 75, so the tire's height is 75 percent of
its width, or 176.25 mm ( .75 x 235 = 176.25
mm, or 6.94 in). The smaller the aspect
ratio, the wider the tire in relation to its
height.
High performance tires usually have a
lower aspect ratio than other tires. This is
because tires with a lower aspect ratio
provide better lateral stability. When a car
goes around a turn lateral forces are
generated and the tire must resist these
forces. Tires with a lower profile have
shorter, stiffer sidewalls so they resist
cornering forces better.
Tire Construction
The R designates that the
tire was made using radial construction.
This is the most common type of tire
construction. Older tires were made using
diagonal bias (D) or bias
belted (B) construction. A
separate note indicates how many plies make
up the sidewall of the tire and the tread.
Rim Diameter
This number specifies, in inches, the wheel
rim diameter the tire is designed for.
Uniform Tire Quality Grading
Passenger car tires also have a grade on
them as part of the uniform tire
quality grading (UTQG) system. You
can check the UTQG rating for your tires on this
page maintained by the U.S. National
Highway Traffic Safety Administration (NHTSA).
Your tire's UTQG rating tells you three
things:
- Tread Wear: This
number comes from testing the tire in
controlled conditions on a government
test track. The higher the number, the
longer you can expect the tread to last.
Since no one will drive his or her car
on exactly the same surfaces and at the
same speeds as the government test
track, the number is not an accurate
indicator of how long your tread will
actually last. It's a good relative
measure, however: You can expect a tire
with a larger number to last longer than
one with a smaller number.
- Traction: Tire
traction is rated AA, A,
B or C,
with AA at the top of the scale. This
rating is based on the tire's ability to
stop a car on wet concrete and asphalt.
It does not indicate the tire's
cornering ability. According to this
NHTSA page, the Firestone Wilderness
AT and Radial ATX II tires that have
been in the news have a traction rating
of B.
- Temperature: The tire
temperature ratings are A,
B or C.
The rating is a measure of how well the
tire dissipates heat and how well it
handles the buildup of heat. The
temperature grade applies to a properly
inflated tire that is not overloaded.
Underinflation, overloading or excessive
speed can lead to more heat buildup.
Excessive heat buildup can cause tires
to wear out faster, or could even lead
to tire failure. According to this
NHTSA page, the Firestone Wilderness
AT and Radial ATX II tires have a
temperature rating of C.
Service Description
The service description consists of two
things:
- Load Ratings: The
load rating is a number that correlates
to the maximum rated load for that tire.
A higher number indicates that the tire
has a higher load capacity. The rating
"105," for example,
corresponds to a load capacity of 2039
pounds (924.87 kg). A separate note on
the tire indicates the load rating at a
given inflation pressure.
- Speed Rating: The
letter that follows the load rating
indicates the maximum speed allowable
for this tire (as long as the weight is
at or below the rated load). For
instance, S indicates
that the tire can handle speeds up to
112 mph (180.246 kph). See the chart on this
page for all the ratings.
Calculating the Tire Diameter
Now that we know what these numbers mean, we
can calculate the overall diameter of a
tire. We multiply the tire width by the
aspect ratio to get the height of the tire.
Tire height = 235 x 75 percent =
176.25 mm (6.94 in)
Then we add twice the tire height to the
rim diameter.
2 x 6.94 in + 15 inches = 28.9 in
(733.8 mm)
This is the unloaded diameter; as soon as
any weight is put on the tire, the diameter
will decrease.
