The letter or letters in the beginning of the tire size indicate the type of service and the type of vehicles that can use these tires. For example, a "P" at the beginning of the tire size means that the tire should be used on passenger vehicles, such as cars, minivans, sport utility vehicles and light duty pickup trucks. This is the most common type of tire size.
If there is a "T" at the beginning of the size, it means that the tire should only be used temporarily when another tire is damaged because it is not suitable for permanent use. If there isn't a letter at the beginning of the size, this means that the tire size uses the metric system. This type of tire size is common for European cars, vans and sport utility vehicles. If there are letters "LT" in the beginning of the tire size, it means that the tire should be used on light trucks, such as those that can carry heavy cargo or pull large trailers.
It is also possible to identify the section width from the tire size by looking at the first three numbers. The width is indicated in millimeters, so divide it by 25.4 to get the value in inches. The two numbers after the section width indicate the tire's profile or aspect ratio. A large number indicates a tall tire sidewall.