235/75R15 — a 28.88 in tire that everyone calls a 29

What the numbers come to

Section width 9.25 in, sidewall 6.94 in, overall diameter 28.88 in. Circumference 90.7 in and 698.4 revolutions per mile. At the half-inch granularity people actually speak in, that is a 29.

The 6.94 in sidewall is the striking figure. It is more sidewall than 225/75R16 carries on a wheel an inch larger — that size manages 6.64 in — and it is why the casing has so much vertical travel available before the rim becomes involved.

The vehicles

This was the standard fitment on a specific generation of North American light truck and compact SUV — the XJ Cherokee, the second-generation Ford Ranger, the Chevrolet S-10 and its GMC twin, and a range of half-ton pickups in their base specification. Those vehicles have largely left the road, and with them the 15-inch light-truck wheel.

Two things kept the size alive anyway. Enthusiast owners of exactly those platforms are still replacing tires. And the trailer industry adopted the size wholesale.

The trailer version is a different tire with the same numbers

Marked ST235/75R15, this is one of the most commonly fitted trailer sizes in North America — boat trailers, utility trailers, small enclosed trailers. The ST prefix is a service type, not a decoration. It designates a casing built and rated to a trailer standard, with a stiffer sidewall and a load rating derived at a higher reference inflation than a passenger casing of the same dimensions.

Geometrically an ST235/75R15 and a P235/75R15 are identical: same 28.88 in, same 9.25 in section, same 698.4 revolutions per mile. In every other respect they are different products. The tire construction codes reference covers what each service prefix changes.

This matters here more than on almost any other size on the site, because it is one of the few codes where a buyer can genuinely end up looking at both types on the same shelf.

What sits above it, in the other notation

The nearest flotation size on the same wheel is 30x9.50R15 at exactly 30.00 in. That is 1.12 in taller than this size and 3.89% up — 0.25 in wider, and enough of a diameter change that an indicated 70 mph would correspond to an actual 72.7 mph, and 100 indicated miles to 103.9.

There is nothing in between. On a 15-inch wheel the metric ladder stops around 28.9 in and the flotation ladder starts at 30, so anyone moving up from this size is not making an incremental change whatever the packaging suggests. Above the 30 the steps run 31x10.50R15, 32x11.50R15 and 33x12.50R15, and the width grows a full inch at every rung.

Downwards, 225/75R15 is 0.59 in shorter at 28.29 in and 215/75R15 is 1.18 in shorter at 27.70 in.

The 235 mm section, across the catalogue

The same 9.25 in of section width appears here at aspect ratios from 40 to 85. The extremes make the point: 235/40R18 stands 25.40 in, and 235/85R16 stands 31.73 in. That is 6.33 in of diameter range from one section width, and this size sits almost exactly in the middle of it.

Compared directly with 235/85R16 — the other tall-narrow light-truck size in this catalogue — the difference is 2.85 in of diameter for the same 9.25 in of tread stance. Wheel diameter accounts for one inch of that; the rest is sidewall.

The tallest metric size on a 15-inch wheel here

Nothing in this catalogue reaches higher than 28.88 in on a 15-inch rim without switching to flotation notation. That is a structural fact about the notation rather than about the tire: metric codes on small wheels run out of aspect ratio before they run out of demand, because a 235 mm casing at 75-series is already carrying 6.94 in of sidewall and going further would need an 85-series or a wider section that 15-inch light-truck wheels were not built to take.

The consequence for anyone reading a fitment list is that a 15-inch wheel forces an early choice between notations, in a way that a 16 or a 17 does not.

Availability, honestly stated

Fifteen-inch light-truck tires are a shrinking category. The tread patterns offered in this size are fewer than they were, and the all-terrain and mud-terrain choices in particular have thinned as manufacturers concentrated their light-truck lines on 16-inch wheels and larger. What remains is stocked because the trailer market keeps the volume up, and trailer-rated casings are not interchangeable in purpose with the light-truck ones they sit next to.

Whether a 15-inch wheel clears a particular vehicle's brakes is a measurement on that vehicle. The figures on this page describe the tire only.