The Basics

• You'll find a long number, usually preceded by a letter, etched onto the side of every tire. This identifier tells you what kind of vehicle the tire is intended for, how much weight it will bear, its measurements and at what speeds and temperatures it can be used.
  
  If the number has no letter in front of it, the tire is a metric size. Metric sizes are used mainly in Europe, but they're also used in the U.S. on pickup trucks and SUVs.
  
  A "P" signifies that the tire is to be used on passenger vehicles such as cars, SUVs, 1/4-ton and 1/2-ton pickup tucks, and minivans. A "T" indicates a temporary spare tire. "LT" is for light trucks and vehicles that carry heavy loads or pull other vehicles, such as SUVs, large vans, and 3/4-ton and one-ton pickups.
  
  If the "LT" is at the end of the number rather than the beginning, the tire is made for vehicles that carry heavy loads, tow other vehicles, use 16.5 inch rims, or it indicates oversized tires made for added traction and load-bearing ability.
  
  If a "C" follows the number, it's a European metric tire made for commercial vans and delivery trucks. An "ST" before the number indicates a special trailer tire that is to be used on general-utility trailers, or those that carry boats or cars. These tires are not to be used on cars.
  
  

Cross-Section Width

• Following the letter, if there is one, are three numbers that give the tire's cross-section width in millimeters. For example, 225 means that the tire measures 225 millimeters across, from the widest part of the outer sidewall to the widest part of the inner sidewall. The measurement is taken when the tire is mounted on a wheel of a particular size.
  
  

Profile

• The next two numbers tell you the ratio of the tire's height to its width. For example, a 50 indicates that the height, measured from the tread to the rim, is 50 percent of the cross-section width.
  
  To convert millimeter measurements to inches, divide the number by 25.4, which is the number of millimeters per inch. For example, if you divide 225 millimeters by 25.4, you arrive at 8.86 inches, which is the cross-section of the tire in the example. To determine the sidewall height, which is 50 percent of the width, divide 8.86 inches by 2, which equals 4.43 inches.
  
  

Type of Construction

• Following the two numbers that give the height-to-width ratio is another letter that gives you information about how the tire is constructed. An "R" means radial, which is the most commonly used type of tire. "D" indicates a diagonal inner construction; these tires are used as spares and for light-duty trucks. "B" means the tire is belted, with reinforcing belts under the treads. Belted tires are generally not used anymore. An "F" following any of these letters means that the tire is a "run-flat." This extra designation isn't mandatory and might not be marked on your tires.
  
  

Diameters

• The next two numbers give the tire and wheel diameters, which are to be used to match the tire to the wheel. These "inch-rim" sizes run from 8 to 28 and are the most commonly used for minivans, light pickups, cars and SUVs. Half-inch measurements are less commonly used, but are still found on tires made for heavy-duty trucks and trailers, and commercial vans.
  
  

Service Description

• The final two numbers and one letter indicate the tire's load-bearing capacity and speed rating, which have been mandatory since 1991. The numbers are assigned according to an index, wherein the lowest rating, 71, equals 761 pounds, and the highest rating, 110, equals 2,337 pounds. The letter is assigned according to the tire's maximum speed capability. For example, an "L" indicates 75 mph or 120 kph, while a "V" denotes 149 mph or 240 kph.