Frequently Asked Questions
The difference between the tire sizes 225/40R18 and 215/40R18 more often than not lies in their width
and sidewall top.
Width: the first wide variety inside the tire length shows the width of the tire in millimeters. So, a
225/40R18 tire is 225 mm extensive, whereas a 215/40R18 tire is 215 mm wide. The difference is 10 mm,
with the 225 being wider.
Sidewall top: the second variety is the component ratio, which is the peak of the sidewall as a
percentage of the width. For each size, the aspect ratio is 40%. However, because the widths are
different, the actual sidewall heights will differ. The sidewall peak of a 215/40R18 tire is 86 mm
(forty percent of 215 mm).
So, while evaluating 225/40R18 to 215/40R18, the former may have a slightly taller sidewall and be
wider, affecting the overall diameter of the tire and potentially the speedometer reading if swapped
without recalibration.
For the specific height of the 215/40 tire, the sidewall top is 86 mm, as referred to above; that's the tire’s peak from the wheel rim to the brink of the tire. The general diameter of the tire will be the rim diameter (18 inches transformed to mm) plus twice the sidewall top, seeing that there’s sidewall on each aspect of the rim.
