The tyre’s section width is measured on an ideal width of wheel, for which it has been designed to operate. The measurement distance in millimeters is from the maximum width of the tyre’s mid point of its sidewall to the same point on the opposite sidewall.
This is the height of the tyre’s sidewall expressed as a percentage of the tyre’s section width. In the example, the height of the tyre’s sidewall is 40% of the tyre’s section width or 245 x 40% = 98mm.
The tyre’s rim diameter is measured in inches taken from the wheel flange where the tyre is seated (bead seat area) to the same point on the opposite side.
