Tire size is an alphanumeric figure that lists a car tire's dimensions. This figure is part of the tire code, which is found on a tire's sidewall or doorjamb placard. Tire size codes include tire class, width, aspect ratio, construction and wheel diameter. Here is a breakdown of each section.
Tire class is typically listed as a P or an LT. The P, which is shown in the graphic above, means that the tire is suited for a passenger vehicle. LT indicates the tire belongs on a light truck or SUV. Less common tire class codes include ST for special trailers or T for spare tires.
The tire width, also known as section width, measures how wide a tire is from sidewall to sidewall. This three-digit number, listed as 225 in the image, is measured in millimeters and is usually sandwiched between the tire class and aspect ratio.
The aspect ratio measures the height of a tire's sidewall. This two-digit number is preceded by a slash mark and follows the tire width. If the aspect ratio is listed as 60, that means the tire's height is equal to 60 percent of its width. A low tire ratio typically indicates a rough ride but improved cornering ability.
Tire construction indicates the internal construction of a tire and is marked with a letter after the aspect ratio. Radial construction is the most common construction and is represented by a letter R as seen in the image above. Other letters include B for bias belt assembly or D for diagonal build. A tire that has a cross ply construction doesn't feature a letter.
Wheel diameter lists the wheel measurement from end to end and indicates the size of the wheel the tire is supposed to be mounted on. This is usually the last number listed at the end of the tire size code. For example, the graphic at the beginning of the article shows that the tire should be mounted on a 17-inch wheel.