Tire Sizing Systems
Numeric Sizing System:

This first system developed for tire sizing was used until the late 1960s, but provided only the cross section width of the tire and the rim diameter in inches. If the section width ended in zero (e.g., 7.00-14 or 7.50-14), the tire had a common aspect ratio of about 92. For section widths not ending in zero (e.g., 8.25-15), the tire was considered "low profile" with an aspect ratio of about 82

Alpha-Numeric Sizing System:

In 1968, a new concept was introduced worldwide. The Alpha-Numeric sizing system is a load-based system where tires are designated by their load-carrying capacity and aspect ratio. The first letter is the load and size relationship, with letters ranging from A to N. The lower the letter, the smaller the size and, of course, the lower the load-carrying capacity of the tire

P-Metric Sizing System:

To accommodate the smaller tires used on compact cars, the P-Metric (Passenger Metric) system was created in 1976. The maximum inflation pressures of P-Metric tires were raised for lower rolling resistance. The P-Metric system is widely used by domestic tire manufacturers.

Metric Sizing System:

Because Europe primarily uses the metric system of measurement, the Metric sizing system was developed. It is essentially a conversion of the Numeric system. Section widths are notated in millimeters instead of inches. Originally, tires not identified with an aspect ratio were assumed to be 82-series. When 60- and 70 series tires appeared, the aspect ratio was added to the nomenclature, similar to the P-Metric system.

ISO Metric Sizing System:

The International Standards Organization (ISO) Metric system combines the Metric system with a service description. The service description provides the load index along with the speed rating symbol.

Millimetric Sizing System:

The Millimetric sizing system is similar to the Metric system except that the rim diameter is also represented in millimeters.