31x10.50R15 — the size with an aspect ratio nobody prints

Where it turns up

Two vehicles in the catalogue list 31x10.50R15 as a common upgrade: the Jeep Cherokee XJ and the Jeep Wrangler TJ. Neither ships it. Both ship 15-inch metric sizes well under 29 in — the XJ on 215/75R15, 225/75R15 and 235/75R15, the TJ on the first two of those plus a 16-inch option.

The number the tire does not tell you

A metric code hands you the shape directly. The middle number of 235/75R15 is the aspect ratio, and it says the sidewall is 75% of the section width.

A flotation code gives you diameter and width instead, and leaves the shape to be reconstructed. Subtract the wheel from the diameter, halve it, and the sidewall here is 8.00 in. Divide that by the 10.50 in of section width and the aspect ratio comes out at 76 — a taller-shouldered tire than a 235/75R15, from a code that never mentions a percentage.

This matters when a flotation size is cross-shopped against a metric one. Two tires with the same stated diameter can have very different aspect ratios, and the one with the higher figure has more sidewall relative to its tread. The code hides the comparison rather than making it.

A ladder that climbs an inch in both directions at once

The 15-inch flotation family is unusually regular:

Size	Diameter	Section width	Sidewall
30x9.50R15	30.00 in	9.50 in	7.50 in
31x10.50R15	31.00 in	10.50 in	8.00 in
32x11.50R15	32.00 in	11.50 in	8.50 in
33x12.50R15	33.00 in	12.50 in	9.00 in

Every step adds exactly one inch of diameter and exactly one inch of section width. Nothing in metric notation behaves like that, because a metric step changes the aspect ratio's base as well as the width.

The regularity ends at the top. 35x12.50R15 stands 35.00 in on the same 12.50 in of width as the 33, so the last rung adds two inches of height and no width at all.

The identical code on a 16-inch wheel

31x10.50R16 is also 31.00 in tall and also 10.50 in wide. The two are the same tire in the only two dimensions their codes state.

They are not the same shape. The 16-inch version has 7.50 in of sidewall against 8.00 in here, and a derived aspect ratio of 71 against 76. Half an inch of sidewall per side moved into the wheel, with the outside of the tire held constant — the plainest possible illustration of what a wheel diameter change actually does when the tire size compensates.

What a 15-inch wheel still has

Every catalogued size on a 15-inch wheel taller than 28.88 in is a flotation size. The metric options stop at 235/75R15; above that, the only codes still produced in volume are the ones with an x in them.

That is the practical state of the 15-inch wheel. It survives on older solid-axle vehicles and in the flotation sizes that were designed around it, and it has attracted no new metric size development for a long time.

What the step is worth on an XJ or a TJ

Because both vehicles ship small 15-inch metric sizes, this upgrade is a large one in percentage terms even though 31 in sounds modest.

• From 235/75R15 at 28.88 in: +2.12 in, 7.35%. An indicated 70 mph is an actual 75.1 mph.
• From 225/75R15 at 28.29 in: +2.71 in, 9.59%, and 76.7 mph.
• From 215/75R15 at 27.70 in: +3.30 in, 11.93%, and 78.3 mph. A hundred indicated miles is 111.9 real ones.

Those percentages come from the base tire being small rather than from this one being large. Eleven point nine per cent on a Cherokee running 215/75R15 is the same 3.30 in that would barely register as a rounding change on a full-size truck already standing at 34 in.

The steps from here

Going up one rung to 32x11.50R15 is +1.00 in and 3.23%. Two rungs, to 33x12.50R15, is +2.00 in and 6.45% — an indicated 70 mph becomes an actual 74.5 mph. Reaching 35x12.50R15 is +4.00 in and 12.90%, or 79.0 mph at an indicated 70. Going down to 30x9.50R15 is −1.00 in and 3.23%, and it takes an inch of section width with it as well as the inch of height.