The numbers in the tire size refers to different parts of the tire. The first number (205) identifies the section width , or the cross section , in millimeters. Typically following the section width is the sidewall aspect ratio or profile (55).
The next two numbers after the width of the tire show the sidewall height in the percentage of the tire’s width. So if the tire markings indicate P225/45R17 91V, it means that the height of the sidewall is 225 multiplied by 0.55 or 101.25 millimeters.