Chapter 12 Driveline and Wheel Components 265
Tire Ratings
Modern tire ratings are designated by a system that
uses a series of numbers and letters. The ratings are
stamped into or molded into the tire’s sidewall. An
example of the rating that would be found on the sidewall
of a typical tire is given in Figure 12-38.
The letter P designates the tire as a passenger car tire,
Figure 12-39. Other letters used in this position are LT for
light truck, T for temporary (such as a space saver spare),
and C for commercial (large trucks and construction vehi-
cles). On tires that are suitable for both cars and trucks, this
letter is not used.
The number 205 is the section width, which is
measured at the tire’s widest point. The section width is a
good indication of tread width, and is generally considered
to be the tire size. The section width ranges from about 175
on small tires to about 235 on large tires.
The number 70 is the aspect ratio, or the relationship
of the tire section width to its height, Figure 12-39. Low
numbers indicate a wide, low tire. The larger the number,
the taller the tire.
The letter R indicates a radial tire. Other letters that
could appear here are B and D for bias and belted tires.
The number 15 is the rim size in inches. The most
common rim sizes are 13, 14, and 15. A few vehicles have
12-, 16-, or 17-inch rims, but these are uncommon.
The number 95 is the load index. The load index is an
assigned number that corresponds to the amount of axle
weight the tire can carry. In this case, a 95 load index indi-
cates the tire can support 1521 lb. (690 kg). Load indexes
usually range from 75–110.
The letter S is the speed rating. The speed rating
indicates the maximum speed at which the tire should be
operated. Speed rating letters range from B, with a maxi-
mum speed of 31 mph (50 km/h), to Z, with a maximum
speed of 149 mph (240 km/h). This number is also
eliminated on some tires.
Tire Quality Ratings
As standardized by the United States Department of
Transportation (DOT), tires are graded according to the
uniform tire quality grading system. The tire grades are
stamped on the sidewall of the tire. This grading system
establishes several quality grades, including the tempera-
ture resistance rating, the traction rating, and the tread
wear rating.
The temperature resistance rating rates the tire’s
resistance to heat generation. Note that this grade is the
resistance to generating heat, not its resistance to the heat
Stabilizer
belts
Radial cord
body plies
Figure 12-37. Typical construction of a radial tire. Sidewall plies
are parallel with each other and at right angles to tread center-
line. Belts are under tread area. (Firestone)
Tire
sidewall
Figure 12-38. Tire ratings are molded or stamped on the tire’s
sidewall. All-season tires are generally marked with “M+S,”
which means they are mud and snow rated. (Buick)
P 205 / 70 R 15 95S
Tire type
P–Passenger
T–Temporary
LT–Light truck
Load index
Speed symbol
Section
width
(millimeters)
205
215
Etc.
Rim
diameter
(inches)
14
15
16
Aspect
ratio
(Section height)
(Section width)
60
70
Construction type
R–Radial
B–Bias–Belted
D–Diagonal (bias)
Section height
Section
width
Figure 12-39. Tire sidewall markings and their meanings.
(Buick)