Most tires are made using one of three primary sizing systems:
The absence of a letter designation before the tire size indicates that tire was constructed according to European standards set in place by the European Tyre and Rim Technical Organization (ETRTO).
A “P” before the tire size designates a p-metric tire. P-metric tires are constructed according to American standards set by the Tire and Rim Association (TRA).
LT-metric (light truck) tires are used primarily on ¾-ton and 1-ton vehicles. With there being a difference in load carrying capacities, LT-metric tires cannot be replaced by Euro-metric or P-metric tires. P-metric and Euro-metric tire-sizing systems are intended for passenger vehicles.
The TRA and the ETRTO both use different calculations to determine a tire’s maximum load carrying capacity indicated by a two or three digit code, called a load index, found after the size description. While two tires may bear the same load index, p-metric and euro-metric will be different in actual load carrying capacity. In fact, euro-metric tires typically have a higher load carrying capacity than p-metric tires of the same size.
Maintaining proper load carrying capacity helps provide the desired handling, braking, and cornering performance intended by the vehicle manufacturer.
When replacing tires, it is important to be sure to install tires that meet or exceed the vehicle’s load capacity. For this reason, it is very easy to replace p-metric tires with euro-metric tires. However, because of the lower load carrying capacity, care must be taken when replacing euro-metric tires with p-metric tires. In certain situations, you can increase the air pressure in a p-metric tire to enable it to carry the same load as a euro-metric tire.
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