285/65R18 — the middle of a three-rung 285 tower

Two vehicles list 285/65R18 as a common upgrade — the 200-Series Land Cruiser and the twelfth-generation F-150 — and neither ships it.

Five points of aspect on a 285 is always 1.12 inches

Size	Aspect	Diameter	Sidewall
285/60R18	60	31.46 in	6.73 in
285/65R18	65	32.59 in	7.29 in
285/70R18	70	33.71 in	7.85 in
285/75R18	75	34.83 in	8.42 in

Four rungs, and every gap in the diameter column is 1.12 in. The figure comes straight out of the width: five per cent of 285 mm is 14.25 mm of sidewall, doubled for the two sides is 28.5 mm, which is 1.12 in. The wheel plays no part in it, so the same step applies to a 285 on any rim.

Because the steps are fixed in inches and not in percentages, they get proportionally smaller as the tower climbs. From the 60 to this size is 3.57%; from here to the 70 is 3.44%; from the 70 to the 75 is 3.32%.

Two hundredths from a size on the same wheel

265/70R18 measures 32.61 in. That is 0.02 in above this tire, 0.06%, and it arrives from a different direction entirely: 0.79 in narrower with 7.30 in of sidewall against this size's 7.29 in.

The two casings are nearly identical in depth and clearly different in width, and they stand at the same height because the narrower one uses a taller aspect ratio to get there. Anyone comparing the two is choosing 20 mm of section width and nothing else.

The flotation option, and what it costs in width

33x12.50R18 is 33.00 in — 0.41 in and 1.27% above this size — and 1.28 in wider at 12.50 in. An indicated 70 mph becomes an actual 70.9 mph.

That is a small change in height for a large change in stance, and it is the usual pattern when a metric truck size is set against the flotation code nearest to it. Where the two notations line up and where they diverge is covered in the metric versus imperial reference, and this size's narrower neighbour is treated in the 275/65R18 comparison.