235/85R16 vs 285/75R16 — height and width are separate axes

Most comparisons on this site put a smaller tire against a bigger one. This pair puts two tires of roughly the same height against each other and lets the width do all the talking.

The 85-series is the unusual one

235/85R16 is a survivor from an era when aspect ratios of 80 and above were ordinary. Its sidewall is 7.86 in tall on a casing only 9.25 in wide — proportionally, the sidewall is nearly as tall as the tire is wide. That produces a tire 31.73 in in diameter out of very little rubber, and it is why the size persisted in utility, ambulance, expedition and overland use long after the passenger market abandoned tall profiles altogether.

285/75R16 gets to 32.83 in a completely different way: a much wider casing, 11.22 in, with a shorter sidewall proportionally at 75 but a taller one absolutely at 8.42 in.

What the delta actually is

Height: 1.10 in, or 3.47%. Width: 1.97 in, or 21.28%.

Read those two lines together and the point of the page is made. In inches, the width gap is nearly twice the height gap. As a proportion of the dimension being changed, it is six times larger. Two tires that a casual glance would call "both about 32 inches" differ by nearly two inches of casing, which is more than the entire diameter gap in most of the pairings on this site.

Which one the vehicle notices

Both, in different places.

• The 1.10 in of height is worth 3.47% at the instruments, and half of it lands under the axle.
• The 1.97 in of width produces nothing at all until the tire turns, at which point it decides whether the assembly clears the inner fender, the frame rail and the steering components.

That is the substance of the "diameter versus width" question, and it is why the two dimensions cannot be traded against each other even though a table shows them in adjacent rows.

The narrow-and-tall trade

A 235/85R16 puts a tall tire under a truck without widening the track. 9.25 in of nominal section reaches 31.73 in of diameter; the 285 needs 11.22 in to reach 32.83 in. Nearly two inches of casing separate two tires 1.10 in apart in height, and a narrower casing is specified against a narrower measuring rim, so the wheel side of the decision follows the same direction as the tire side. The specific widths each is approved for are a per-tire figure on the manufacturer's sheet and are not carried here. For anyone whose limiting dimension is width rather than height, that narrowness is the whole reason the size still exists.

It costs something. A 9.25 in casing has less footprint area at a given inflation pressure than an 11.22 in one, and its aspect ratio makes the sidewall taller and therefore more flexible in cornering. Neither of those is a verdict on either tire; they are consequences of the shapes.

The 17-inch equivalent of the same idea

There is a modern echo of this pairing one wheel size up: 255/80R17 against 285/70R17 reaches nearly the same diameter two different ways, with a 30 mm width gap and an 80-series sidewall doing the work. If this page's argument interests you, that one is the same argument with the numbers rearranged.

What they get called

Rounded to the nearest half inch — which is the granularity flotation sizes are sold in — the narrow one is a 31.5 and the wide one is a 33. In conversation they are usually described as "a 32" and "a 33", and the gap between those two labels is far wider than the 1.10 in that separates the tires. Names round; casings do not.

The rounding is not symmetrical either. A 31.73 in tire and a 32.83 in tire are 1.10 in apart, but one of them rounds down to a label and the other rounds up to a different one, so a gap of about three per cent is reported in conversation as a gap of a whole inch and a half. That is a property of the names, not of the tires.

Reading them on a shelf

Both codes are sold with and without an LT prefix and across a wide band of load indexes, and the narrow size in particular is common in high load ranges because its buyers are usually carrying weight. That is a property of the tire's construction rather than its shape, and it will not appear in any dimension above. The load range reference covers what the marking means; your door-jamb placard covers what your vehicle requires.

Whether either fits is a question about a specific truck with specific wheels, measured at full lock. Nothing here answers it.