235/45R18 tire size — dimensions and odometer consequences

Premium mid-size saloons and estates carry this size in their base and mid wheel options — 3-series and 4-series, A4 and A5, C-class, Mazda6 in upper trims.

One hundred recorded miles, four alternatives

An odometer counts wheel revolutions and multiplies by a fixed distance per revolution. Change the tire and the multiplier is wrong by exactly the diameter ratio. Taking this size as the calibration point:

Fitted instead	Diameter	Odometer reads 100	Difference
225/45R18	25.97 in	98.7 real miles	1.35%
215/55R17	26.31 in	99.9 real miles	0.06%
245/45R18	26.68 in	101.3 real miles	1.35%
255/45R18	27.04 in	102.7 real miles	2.69%

The 215/55R17 row is the one to notice. A tire on a wheel an inch smaller, 0.79 in narrower, with half an inch more sidewall, and the odometer cannot tell the difference — 26.31 in against 26.33 in.

The two 45-series rows either side are symmetric at 0.35 in and 1.35% because a 10 mm width step at a fixed aspect ratio and wheel is a fixed quantity.

The 235 mm section, at three aspect ratios on this wheel

235/40R18 is 0.93 in shorter at 25.40 in; 235/50R18 is 0.93 in taller at 27.25 in. This size sits exactly between them, which makes it the natural reference point for the whole 18-inch 235 mm range.

What is not in the code

A 1.35% error either side sounds negligible, and over a single journey it is. Over a service interval it is not: at 10,000 recorded miles the same error is 135 miles, which is the difference between a schedule met and a schedule missed by a fortnight of ordinary driving.

The 215/55R17 row has a second use for the same reason. A car delivered on this size and given a 17-inch winter set in 215/55R17 keeps its odometer within 0.06% and its speedometer with it, so the two sets can be swapped twice a year without any recorded distance drifting between them. Very few winter downsizes land that close; most cost a per cent or more in each direction.