255/40R18 tire size — two codes that print the same diameter

Performance saloons and coupes, very often as the rear half of a staggered set, and widely in the aftermarket. Nothing at 40-series on an 18-inch wheel in this catalogue is wider.

The same number, twice

255/35R19 also measures 26.03 in. Both codes carry a 255 mm section, so both are 10.04 in wide. The 19-inch version has 3.51 in of sidewall and the 18-inch version 4.02 in.

Add an inch of wheel and remove five points of aspect ratio, and at this particular section width the two changes cancel almost exactly. They do not cancel at every width — the amount of sidewall five points represents depends on how wide the casing is — which is why this is a property of 255 mm specifically rather than a general rule.

The two sizes agree to 0.02%, which is inside the manufacturing tolerance of a real tire and far inside anything an instrument would register.

The 40-series family on this wheel

225/40R18 measures 25.09 in, 235/40R18 measures 25.40 in and 245/40R18 measures 25.72 in. This size sits 0.94 in above the narrowest of them, having gained 1.18 in of section width to get there. Each 10 mm step is worth 0.31 in of height.

Its nearest neighbours

Size	Wheel	Diameter	Difference
225/45R18	18 in	25.97 in	0.06 in
215/60R16	16 in	26.16 in	+0.13 in
225/50R17	17 in	25.86 in	0.17 in
215/55R17	17 in	26.31 in	+0.28 in

The 225/45R18 pairing is the one that matters in practice: same wheel, 1.18 in less section width, essentially the same sidewall and the same standing height. It is the front-and-rear pair of a great many staggered sets.

Why 255 mm and not some other width

The cancellation described at the top has an exact answer, and it is worth working out because it explains why this pairing is a coincidence rather than a rule.

Five points of aspect ratio produce sidewall equal to five per cent of the section width, on each side of the tire. For the two sides together to come to exactly the 1.00 in that an extra inch of wheel adds, the section width has to be ten per cent of 10.00 in — that is, 254 mm.

The catalogue has no 254 mm size, because tire widths are sold in 10 mm steps. It has 255 mm, which misses the exact figure by a single millimetre and produces the 0.02% residue between 255/40R18 and 255/35R19.

Try the same trick a step either side and it fails visibly. At 245 mm the five-point step is worth less than an inch, so the taller-wheel size comes out shorter; at 265 mm it is worth more, so the taller-wheel size comes out taller. Only here do the two operations land on top of each other.