The first three-digit numeric portion identifies the tyre's "Section Width" (cross section) in millimeters.
225 /50R16 91S
The 225 indicates this tyre is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel. This measurement is also referred to as the tyre's section width. Because many people think of measurements in inches, the 225mm can be converted to inches by dividing the section width in millimeters by 25.4 (the number of millimeters per inch).
225mm / 25.4 = 8.86"