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What is the difference between P-metric and Metric car tires?

Consumer Reports News: August 29, 2011 09:23 AM

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In our Tire Talk forum, some of our readers have questioned the interchangeability of P-metric and Metric tire size designations. But answering this question is like asking how to solve the national debt crisis. In other words, there are many answers, and some might be more practical solutions than others.

There are two major size designations commonly used for passenger car tires, called P-metric and Metric sizing. If you see a “P” in front of the tire size displayed on a tire’s sidewall it is a P-metric tire. If there’s no P, it’s a Metric tire (also called Euro-metric or Hard-Metric). In the past, P-metrics were the most common size designation found on domestic cars, minivans, and light-duty pickups and SUVs. Foreign cars generally came with Metric tires. These days, however, you may find P-metric and Metric tires on all types.

So the oft-asked question is: Can you use P-metric and Metric tires interchangeably when buying replacement tires?

Even within the industry, there are conflicting view points. To illustrate, let’s look at the differences between a P215/60R16 and 215/60R16 tire. Physically, they have about the same dimensions and appear to fit interchangeably, but you will notice a difference in the two- or three-digit number following the size called the load index. The higher the load index, the greater the load-carrying capacity of the tire. The P215/60R16 has a load index of 94, corresponding to a maximum load of 1,477 lbs at 35 psi; the 215/60R16 tire has a load index of 95 for a maximum of 1,521 lbs at 36 psi.

Why the difference? Both types use different load and inflation formulas; as a result, a metric tire has a slightly greater load index and higher load-carrying capacity reserve than its P-metric counterpart. (Learn how to decode tire size and other data.)

Hence, if your car came with P-metric tires, you can replace them with P-metric or Metric tires. The Metric tire will have greater load capacity reserve than what came on the car as original equipment and is an acceptable substitute. A number of tire manufacturers allow for this substitution.

However, since P-metric tires have a lower load capacity reserve than their Metric counterparts, you shouldn’t use P-metric tire size counterpart of the Metric tire that came on your car as original equipment. Some astute forum readers may challenge this point by noting the P-metric tire’s pressure can be adjusted to provide an equivalent maximum load capacity of a Metric tire. This is true within certain limitations, but the conversion is best left to the professional tire dealer who has access to a tire fitment guide for proper tire replacement.

Overall, the best practice is to replace tires with the same size designation and load index that originally came on your car. Any questions about your tire’s details can be found on the placard mounted on the door jamb of most cars. Some older vehicles may have the placard in the glove box, behind the fuel filler cover, or the inside of the truck lid.

Finally, don’t mix P-metric and Metric tires on a car as it could compromise handling, braking, and cornering performance.

Now for the debt crisis…

Gene Petersen

   

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