The description of the tire size, in general, is formed by the nominal measures of the section width and the internal diameter of the tire, which may include the nominal aspect ratio (series) between the two measures.

Example:
225 / 45R17
225 - Indicates the nominal width of the tire expressed in millimeters.
45 - Indicates the percentage quotient between the height of the section and the nominal width of the tire approximately equal to 45.
17 - Indicates the inner diameter of the tire expressed in inches.
R - The letter "R" indicates the type of construction "Radial".