Thera are a number of different systems in use today that describe the various tire sizes used on performance, passenger and light truck vehicles. An attempt is currently underway to develop a worldwide system, but presently you must be familiar with several different systems.
The 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 has 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.
In 1968, a new concept was introduced worldwide. The Alpha-Numeric Sizing System is a load-based system where tires are designed by their load-carrying capacity and aspect ratio. The first letter is the load and size relationship, with letter, the smaller the size and, of course, the lower the load-carrying capacity of the tire.
To accomodate 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 manufacturer
Because Europe primarily uses the metric system of measurement, the Metric Sizing System was developed. It is essentially a conversion of the Numeric Sizing System. Section widths are notated in milimeters 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.
The International Standard Organization (ISO) Metric System combines the Metric System with a service description provides the load index along with the speed-rating symbol.
The Milimetric Sizing System is similar to the Metric Sizing System except that the rim diameter is also represented in milimeters. These rim diameters are unique in size and can only be used in combination with each other.
Similar to the Numeric Sizing System for cars, it lists the section width in inches, construction type, rim diameter in inches, plus the light truck designation
Similar to the P-Metric System, except the P is replaced with the LT (Light Truck) designation. Also, LT-Metric and P-Metric tires differ in construction.
Tha same as the Light Truck Numeric System with tire diameter added to the front.