If you have ever looked at the side of a tire before then chances are you have noticed (at least in passing) that there are some letters and numbers grouped together in some kind of order. Well, that group of numbers and letters is actually an alphanumeric code that has been mandated by the International Organization for Standardization (ISO) to allow people to know the specifications of a tire with relative ease, regardless of the language used. Understanding tire codes is rather simple as long as you know the secret. To help make the process a little easier, use this code (which could easily be found on any car) as an example:
- Vehicle class. In our example code, the letter "P" is used. This indicates that the tire is designed for a passenger car. However, it could also be an "LT" (for light truck), "ST" (for special trailer), or "T" (for temporary, usually seen on a donut spare).
- Three-digit number. The three-digit number after the vehicle class is how wide the tire is. This is usually measured at the widest point of the tire from both edges, and is called the nominal section width. In the example above, this tire has a 215 mm nominal width.
- Slash. The slash is nothing more than an indicator showing that there is a break in the code. Typically used to separate two different sections of code from each other for an easier time of reading and understanding.
- Two or three digit number. Following the slash is usually a two or three digit number that is used to indicate the height of the sidewall. This height is typically represented in a ratio called an "aspect ratio" that is the height to the total width of the tire. For example, in this code that would be 65 percent (or 139.75 mm). If the number is higher than two hundred then it is assumed that it is the diameter of the entire tire in millimeters.
- Letter. The letter that follows the two or three digit combination is purely optional used to indicate how the tire was constructed, but there are three common letters used. In the example provided you will see that there is an "R" which indicates that this tire is a radial tire. The other two commonly used letters are "B" (for belt) and "D" (for diagonal). If you don't see a letter it means that the tire is a cross ply tire.
- Two-digit number. Next comes another two digits, which indicate the diameter (in inches) of the wheel that the tire is designed to fit. In the example provided here, the tire was designed to fit a 15-inch diameter wheel.
- One or two digit and letter combo. The last three alphanumeric combination found in our example (and usually found on most tires) is indicative of the load and speed rating for the tire. Our example was rated at 95, which means that it can carry a maximum weight of 690 kilograms (or 1,500 pounds) per tire, and can go up to 130 miles per hour safely.