Figure 8-30.Radial tire construction.
A major disadvantage of the radial ply tire is that it
produces a harder ride at low speeds. The stiff tread
does NOT give or flex as much on rough road surfaces.
There is important information on the sidewall of a
tire. Typically, youll find UTQG (Uniform Tire
Quality Grading) ratings for treadwear, traction, and
temperature. Also, you will also find the tire size, load
index and speed rating, and inflation pressure. It is
important that you understand these tire markings.
Tire size on the sidewall of a tire is given in a
letter-number sequence. There are two common size
designations (fig. 8-31)alphanumeric (conventional
measuring system) and P-metric (metric measuring
The alphanumeric tire size rating system, as shown
in figure 8-31, uses letters and numbers to denote tire
size in inches and load-carrying capacity in pounds.
The letter G indicates the load and size relationship.
The higher the letter the larger the size and
load-carrying capability. The letter R designates the
radial design of the tire. The first number "78" is the
aspect ratio, also known as height-to-width ratio. The
last number "15" is the rim diameter in inches.
The P-metric tire size identification system, as
shown in figure 8-31, uses metric values and
international standards. The letter P indicates a
passenger vehicle (T means temporary and C means
commercial). The first number "155" indicates the
section width in millimeters measured from sidewall to
sidewall. The second number "80" is the aspect ratio,
also known as height-to-width ratio. The letter R
indicates radial (B means bias belted, D means bias-ply
Truck tires are sometimes marked with the
designation LT for "light truck" before the size.
The ASPECT RATIO or height-to-width ratio in
the tire size is the most difficult value to understand.
Aspect ratio is the comparison of the height of a tire
Figure 8-31.Tire size designation numbering systems.