285/70R18 — past the 33 without changing notation

The third-generation Toyota Tundra lists 285/70R18 among its common upgrades. Nothing in the catalogue ships it.

It is already taller than the 33

33x12.50R18 measures 33.00 in exactly. This size measures 33.71 in — 0.71 in and 2.10% above it, on a casing 1.28 in narrower.

That inverts the usual arrangement. On most wheel diameters the metric near-33 falls a little short of the flotation one and the argument is about how short; here the metric size overshoots and the flotation size is the smaller of the two. A truck fitted with these and then offered "a set of 33s" would be going down.

The 18-inch ladder from here

Size	Diameter	Against 285/70R18	Section width
275/65R18	32.07 in	−1.63 in (4.85%)	10.83 in
285/65R18	32.59 in	−1.12 in	11.22 in
265/70R18	32.61 in	−1.10 in	10.43 in
33x12.50R18	33.00 in	−0.71 in	12.50 in
275/70R18	33.16 in	−0.55 in	10.83 in
285/70R18	33.71 in	—	11.22 in
35x12.50R18	35.00 in	+1.29 in (3.83%)	12.50 in
285/75R18	34.83 in	+1.12 in	11.22 in

Six catalogued sizes sit within 1.63 in below it and only two above. This is near the top of what the 18-inch wheel offers, and the two codes above it go about it in opposite ways: one adds an aspect step on the same casing, the other adds 1.28 in of width and switches notation.

What the Tundra actually gains

The Tundra's factory 18-inch fitment is 275/65R18 at 32.07 in, so the step to this size is 1.63 in. As a proportion of the tire being left behind that is 5.09%: an indicated 70 mph becomes an actual 73.6 mph and a hundred indicated miles becomes 105.1. The table above quotes the same gap as 4.85% because it measures every row against this size rather than against the starting one.

The axle rises 0.82 in. Section width goes up 0.39 in — a tenth of what the height change might suggest, because this upgrade is bought almost entirely in aspect ratio rather than in casing width. Which of the three numbers a change is spent on is the part a single percentage never records, and the tire size code reference sets out how each one enters the arithmetic.