Tire Size
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 system).

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 construction).

NOTE

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.

(bead to tread) to the width of a tire (sidewall to sidewall). It is height divided by width. A 80-series tire, for example, has a section height that is 80 percent of the section width.

As the aspect ratio becomes smaller, the tire becomes more squat (wider and shorter). A 60-series tire would be "short" and "fat." whereas an 80-series tire would be "narrower" and "taller."