Three primary sizing systems exist for passenger tires today: P-Metric, European Metric and Millimetric. Each of these systems evolved from the first tire sizing system-the Numeric Sizing system-that is now obsolete. It was developed when all tires had the same aspect ratio, and it provided only the nominal cross section width of the tire and the rim diameter in inches. The following are examples that identify the three sizing systems that are commonly seen today.
P-Metric - The P-Metric sizing system was developed to better align with the European tire sizing system. It provides a better description of the tire size. See examples below.
European Metric - Essentially, this system was a conversion of the Numeric system from inches to millimeters. Aspect ratio appears in the size designation in most cases where it is other than 82.
Light Truck Tire Sizing - Sizing for light truck tires takes the performance requirements of the vehicle, and the tires, into account. Light truck tires have evolved along with the expanded applications of trucks and vans that have grown to be multi-purpose vehicles that we use for work, for recreation or as passenger vehicles. There are three primary light truck tire sizing systems: Light Truck Metric, Light Truck High Floatation and Light Truck Numeric.
Light Truck Metric - This sizing system mirrors the P-Metric system for passenger tires.
Light Truck High Flotation - Light truck high flotation tires have evolved as lower aspect ratio tires became more popular on light trucks. The combination of lower aspect ratios and high flotation yielded better traction on sand and soft soil found in off-road situations.
Light Truck European Commercial Metric - This size system mirrors the European Metric system for passenger tires and is intended for commercial light truck vehicles sometimes referred to as “Euro Vans”. Often these tires are referred to as European C-Type sizes.
Light Truck Numeric - This older system is still widely used, mostly on commercial vehicles.