33x12.50R15 — nine inches of sidewall

Three Jeeps and no factory

33x12.50R15 appears in the upgrade lists of the Jeep Cherokee XJ, the Jeep Wrangler TJ and the Jeep Wrangler JK. It is not a factory size on anything in this catalogue, and given the vehicles involved that is unsurprising: the XJ's own 15-inch fitments top out at 235/75R15 and 28.88 in.

Where the nine inches comes from

A flotation code states the outside diameter directly, so the sidewall falls out of a single subtraction: 33 in of tire, 15 in of wheel, 18 in of difference, 9.00 in above the rim on each side.

Only one 15-inch size catalogued here carries more — 35x12.50R15, at 10.00 in — and the depth is a direct consequence of the wheel being small rather than the tire being tall. A 33 on a 20-inch wheel gets 6.50 in. Same outside diameter, same tread width, two and a half inches less rubber between the rim and the road on each side.

Its aspect ratio is 72, and so is a 35's on a 17-inch wheel

Divide the 9.00 in of sidewall by the 12.50 in of section width and the derived aspect ratio comes to 72.

Now do the same for 35x12.50R17: 35 minus 17 is 18, halved is 9.00 in, over the same 12.50 in of width — also 72. Two sizes, two inches apart in diameter and two inches apart in wheel, with identical proportions.

They are the same shape at different scales, which is a useful thing to know if you have run one and are trying to picture the other. Everything about how the casing is proportioned carries over. The absolute figures do not: the 35 turns 576.2 times per mile against this size's 611.2.

What it replaces

Coming from	Diameter	Step	Change	Section width added
225/75R15	28.29 in	+4.71 in	16.66%	+3.64 in
235/75R15	28.88 in	+4.12 in	14.27%	+3.25 in
30x9.50R15	30.00 in	+3.00 in	10.00%	+3.00 in
31x10.50R15	31.00 in	+2.00 in	6.45%	+2.00 in
32x11.50R15	32.00 in	+1.00 in	3.13%	+1.00 in

The top two rows are why this size has the reputation it does. Moving from a factory XJ tire to this one is a four-inch change in diameter and more than three inches in section width — a transformation rather than an upgrade. An indicated 70 mph on a Cherokee running 235/75R15 becomes an actual 80.0 mph once these are fitted, and a hundred indicated miles becomes 114.3.

The lower three rows show the flotation ladder's regularity in inches: on a 15-inch wheel, each inch of diameter arrives with an inch of section width, so every step is square. The percentages are not, because each one is measured against a different starting diameter — the same 1.00 in step is 3.13% here and would be under 3% on anything taller.

The metric sizes it is not comparable to

There is no near-equivalent metric size on this wheel. The tallest 15-inch metric code in the catalogue is 235/75R15, and this size is 4.12 in above it on a casing 3.25 in wider. Every other wheel diameter in the catalogue has a metric size within a fraction of an inch of its flotation 33 — 285/75R16 is 0.17 in away, 285/70R17 is 0.29 in away, 275/60R20 is 0.01 in away. On a 15-inch wheel there is nothing.

That absence is the single clearest statement of what happened to the 15-inch wheel. Tire manufacturers kept building the flotation sizes that already existed for it and stopped introducing metric ones, so the only route to a tall tire on a 15 is a code written the old way.

The one rung above

35x12.50R15 stands 35.00 in on the same 12.50 in of width — the only larger 15-inch size in the catalogue, and the point at which the ladder stops adding width. It is +2.00 in and 6.06%, turning an indicated 70 mph into an actual 74.2 mph. Its sidewall is 10.00 in, and its derived aspect ratio is 80.

What it does to a recorded distance

Speedometer error gets the attention because it happens in front of the driver. The odometer error is the same proportion applied to every mile, and it accumulates silently.

A Cherokee XJ that left the factory on 235/75R15 and now wears these is under-recording by 14.27%. Ten thousand miles on the instrument cluster is 11,427 miles on the road. Coming from a TJ's 30x9.50R15 the figure is 10.00%, so ten thousand indicated becomes 11,000.

Nothing on the vehicle corrects for that, and nothing records when it started. The figure on the instrument cluster of a thirty-year-old Jeep is the sum of however many tire sizes it has worn, each contributing its own proportion, and no part of that history is recoverable from the number displayed. Buyers read one figure; the tire that produced it changed several times.