Recently I had a customer send me a question asking about the differences between a P-metric listed tire, (i.e. P225/45-17), and one without the P in the size, (i.e. 225/45-17). So I thought this would be a good topic to post a quick blog about.
To understand the differences in these types of tires, you must first realize that tires are designed for a true global market. Bridgestone, Michelin, Yokohama, Goodyear, to name a few, all produce tires for this global marketplace. If they can produce one tire in a given size that meets all the requirements for every market that tire is sold in, it simplifies production,and gives shorter production lead times.
Let's start with passenger tires, or P-metric tires, as they are called. P-metric tires are the tires with the P at the beginning of the tire size, P225/45-17 for example. This type of tire was was introduced in the United States beginning in the late 1970s and eventually replaced bias ply style tires. These tires are intended for use on vehicles primarily used to carry passengers, including cars, station wagons, sport utility vehicles and even light duty pickup trucks. The load carrying capacity of a P-metric tire is based on an engineering formula that takes into account the physical size (the volume of space for air inside the tire) and the amount of air pressure (how tightly the air molecules are compressed). Because all P-metric tires follow the same formula for load, vehicle manufacturers can design new vehicles around existing or new tire sizes created with the new load formula.
Euro-metric sized tires do not have the P at the beginning of the size, 225/45-17 for example. Using metric dimensions to reflect a tire's width began in Europe in the late 60s. Euro-metric sizes were developed over time based on load and dimensional requirements of new vehicles. The tires were developed based on load and size dimensions, along with the vehicle it was intended for use on. While this did give you a tire that fit a particular vehicle's needs perfectly, this approach was not quite as proactive and uniform as creating sizes using a formula. It did work however and created many new sizes that are still in use to this day.
Both P-metric and Euro-metric tires with the same calculation (i.e. P225/45-17 & 225/45-17) are equal in size to each other. The one slight difference is in their load carrying capacity and maximum inflation pressure. Euro-metric tires have a slightly higher load capacity, and can be inflated to a higher pressure. Because P-metric and Euro-metric tires have the same physical size and the same tire performance categories, the two are regarded as equivalent and interchangeable. As long as they are used in axle pairs or sets of four you can use either type on your car. Just continue to follow your vehicle manufacturer's recommended inflation pressures and rotation guidelines.
So you can see there is no real huge difference between these tire types. They were developed to accomplish the same thing at around the same basic time frame. Just in two different parts of the world using two different construction guidelines.
For more information on tire construction and tire types review our library of tire tech articles.