The numbers 215-50-17 and 225-55-17 represent two types of tire sizes for automobiles. Numbers like these are usually inscribed on the tire's sidewall. Each of the three sections that comprises each number represents a bit of information about the tire, which is particularly useful in the event of replacement and installation.
The first section of each number, consisting of three digits, stands for the section width of the tire. This is the measurement in millimeters of the widest point of the tire's outer sidewall to the widest point of its inner sidewall. Thus, a tire with the 215-50-17 inscription has a section width of 215 mm, while a tire with the number 225-55-17 has a section width of 225 mm. In inches, the 215-50-17 tire has a section width of 8.46 inches, while that of the 225-55-17 tire is 8.86 inches.
Following the section-width indicator are two digits that stand for the sidewall aspect ratio, or the tire's profile. This is the ratio of the tire size's sidewall, or section height -- which is measured from the rim to the tread -- to the section width, represented as a percentage. Thus, a 215-50-17 and 225-55-17's tire's section height is 50 percent and 55 percent, respectively, of its section width.
The tire's section height itself can be determined by multiplying the section width by the percentage figure that the sidewall aspect ratio provides. This results in a section height of 107.5 mm, or 4.23 inches, for a 215-50-17 tire; and a section height of 123.75 mm, or 4.87 inches, for a 225-55-17 tire.
The third and last numeric section, composed of two digits, denotes the tire and wheel diameter, expressed in inches, meant for pairing. The numbers 215-50-17 and 225-55-17 mean that the tire bearing this inscription is a 17-inch rubber covering fitted on a 17-inch wheel. The 17-inch tire and wheel is most commonly applied on personal or family automobiles such as cars, minivans and sport utility vehicles.
