What do the Tire Size Numbers mean?
205/55 R16 91W
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 sidewall height, from rim to tread, is 55 % of the section width of 205, or 112.75 millimeters (4.44 inches).
The last part is the internal construction of the tire. R stands
for radial constuction, in which the tire's body plies "radiate"
out from the imaginary center of the wheel. Virtually all tires sold today
are of radial construction.
The number following (16) indicates which wheel diameter this tire is designed to match. Tires have rim diameters expressed in inches.