To accommodate the smaller tires used on compact cars, the P-Metric (Passenger Metric) system was created.
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.
Light Truck Numeric System
Similar to the Numeric system for cars, it lists the section width in inches, construction type, rim diameter
in inches, plus the light truck designation.
Light Truck Metric Sizing System
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.
