235/85R16 — the tall, narrow one

An aspect ratio nothing else here uses

Eighty-five. No other size code on this site prints a higher middle number, and the next one down is 80, which appears exactly once. Every other light-truck fitment on a 16-inch wheel here sits at 70 or 75. Flotation codes print no aspect ratio at all, and one of them works out higher still — a point the aspect ratio reference picks up.

That single digit pair produces a sidewall of 7.86 in on a section only 9.25 in wide — a casing that is very nearly as tall in profile as it is broad. Overall diameter comes to 31.73 in, or 806 mm, with a circumference of 99.7 in and 635.6 revolutions per mile.

The comparison that explains the whole size

Put it next to 265/75R16, the size a buyer would normally reach for at this diameter on this wheel.

265/75R16 stands 31.65 in. This size stands 31.73 in. The difference is 0.08 in — a quarter of one per cent, which is to say nothing at all. A speedometer cannot register it and no arch clearance calculation would care.

The section widths are 10.43 in and 9.25 in. That is 1.18 in of difference, and it is the entire point. Two tires of identical height, one over an inch narrower than the other, on the same wheel.

Nothing else in this catalogue offers that trade at this diameter. It is why the size persists in places where a narrow footprint has a job to do — deep snow, soft ground, and vehicles built to carry weight on a narrow track.

Where the height comes from

The sidewall does all the work. At 7.86 in, this size carries taller sidewalls than 245/75R16 (7.23 in) and 255/70R16 (7.03 in), both of which are wider tires on the same wheel and both of which come out shorter: 30.47 in and 30.06 in respectively.

Against 31x10.50R16, the flotation size nominally at the same place on the ladder, this size is 0.73 in taller and 1.25 in narrower. The flotation notation states 31 in and delivers exactly 31.00 in; this metric code states nothing about its height and delivers 31.73 in.

What the market actually stocks

In practice this size is sold almost entirely as a light-truck casing, marked LT, in the heavier load ranges. It is the standard fitment on a great many ambulance and utility-body chassis, and it has a long-running second life on expedition-prepared full-size and heavy-duty pickups, where the narrow section is chosen deliberately rather than tolerated.

That single-construction reality is unusual. Most of the sizes on this site appear in both passenger and light-truck form, and the two can be hundreds of pounds apart in rated capacity. Here the question rarely arises, but the load index on the sidewall is still the number to read, and the load range and ply rating reference explains how the older ply-rating language maps onto it.

The same section width, five and a half inches apart

235 mm appears throughout this catalogue. The shortest entry is 235/40R18 at 25.40 in. This is the tallest, at 31.73 in. Same tread stance, 6.33 in of difference in how tall the tire stands.

Between them sit nine other 235 mm sizes, and every one of them differs from its neighbours only in aspect ratio and wheel diameter. It is the clearest demonstration in the catalogue that section width and overall diameter are independent quantities — a point the section width vs tread width reference takes further, because section width is not tread width either.

What a narrow section does to the contact patch

For a given load and a given inflation pressure, the area of the contact patch is fixed by physics rather than by the tire's shape: force divided by pressure. What the shape decides is how that area is laid out. A narrow casing produces a long, thin patch; a wide one produces a short, broad patch of the same area.

That is a geometric statement and nothing more. It is the reason this size and 265/75R16 behave differently despite standing the same height, and it is the reason narrow-and-tall remained a deliberate specification long after fashion moved the other way.

The only other survivor of the idea

The 80 mentioned at the top belongs to 255/80R17, and the two sizes make a short and rather lonely list.

255/80R17 stands 33.06 in on 10.04 in of section, with 8.03 in of sidewall. This size stands 31.73 in on 9.25 in, with 7.86 in. Between them they are the whole of the tall-narrow category in this catalogue: one on each of the two wheel diameters that heavy-duty and utility chassis actually use, roughly an inch and a third apart in height, three quarters of an inch apart in width.

Everything else that reaches those diameters does it by getting wider. 33x12.50R16 matches the 17-inch size's height on 12.50 in of casing — 2.46 in more than 255/80R17 and 3.25 in more than this one.

So the two 80-plus-series codes are not a trend that thinned out. They are what is left of a different answer to the same question, kept alive by vehicles that need to put weight on a narrow footprint and are indifferent to how the result looks.