255/60R18 — indicated speed against actual, for nearby sizes

Full-size crossovers and mid-size SUVs — Grand Cherokee, Explorer, Pilot, Traverse — in their 18-inch wheel packages.

The same wheel, four alternatives, one instrument

A speedometer reads in proportion to rolling circumference. Taking a vehicle calibrated for this size:

Fitted instead	Diameter	Indicated 70 reads	Difference
235/60R18	29.10 in	67.8 mph actual	3.14%
245/60R18	29.57 in	68.9 mph actual	1.60%
265/60R18	30.52 in	71.1 mph actual	1.57%
255/65R18	31.05 in	72.3 mph actual	3.34%

Three of those are width changes at a constant aspect ratio and one is an aspect change at constant width. The aspect change moves the reading twice as far as a 10 mm width change does, which is the general relationship at any width.

The 245 and 265 rows are almost symmetric — 1.60% down and 1.57% up — and not exactly so, because the percentage is computed against a base that is itself moving.

Where the alternatives on other wheels land

255/65R17, 255/70R16 and 255/55R19 all measure within 0.02 in of this size. They are the same casing width at four different wheel diameters, and any of them would leave the speedometer untouched.

235/65R18 sits 0.02 in below at 30.03 in, from a narrower casing on the same wheel.

The 18-inch crossover range around it

265/60R18 at 30.52 in and 285/60R18 at 31.46 in continue the 60-series family upwards, and 265/65R18 at 31.56 in and 255/65R18 at 31.05 in continue it by aspect ratio instead. Between 29.10 in and 31.56 in an 18-inch wheel offers eight distinct sizes in this catalogue, which is as densely packed as any part of it gets.

That density is why 18 inches became the default crossover wheel: whatever rolling diameter a platform needs, there is a size that hits it.

Why a real one measures shorter than this

6.02 in is a substantial amount of casing for an 18-inch fitment and it is why the loaded rolling radius of a real tire in this size sits noticeably below the free radius computed here. The overall diameter and rolling radius reference covers the difference, which matters when comparing these figures with a manufacturer's published revolutions-per-mile number.

That gap between computed and measured also explains why two owners can tape-measure the same size and disagree. Tread depth, inflation, load and even how long the vehicle has been standing all move the number a tape returns, and none of them moves the number the code specifies.