What is 31x10.50R15 in metric?

The one conversion on this site with no answer

Most flotation-to-metric conversions land near something real. This one does not. 31x10.50R15 works out to 267 mm of section on a 76-series sidewall — 267/76R15 — and the metric 15-inch catalogue has nothing within two inches of it:

• 235/75R15 — 28.88 in, 2.12 in short
• 225/75R15 — 28.29 in, 2.71 in short
• 215/75R15 — 27.70 in, 3.30 in short

The gap is not a rounding problem. It is an absence: metric light-truck sizes on 15-inch wheels were largely discontinued as vehicles moved to larger rims, and the tall ones went first.

Change the wheel and the answer appears immediately

Allow a different rim diameter and 31.00 in becomes easy to match. 255/70R17 measures 31.06 in and 255/65R18 measures 31.05 in — both within a rotation per mile of the flotation size, both readily available. What cannot be done is staying on a 15-inch wheel.

That is the practical content of this conversion: the question "what is this in metric" is really "what can I buy instead", and on a 15-inch rim the honest answer is another flotation tire.

What the two-inch shortfall would do

Fitting 235/75R15 where a 31x10.50R15 was calibrated puts the speedometer 6.85% out: an indicated 70 mph becomes 65.2 mph, and the vehicle sits 1.06 in lower at the axle. That is not a substitution anyone would make by accident, and it is worth quantifying precisely because the two sizes appear near each other on cross-reference charts that rank by rim diameter rather than by height.

Why the aspect ratio comes out at 76

Flotation notation states the diameter, so the sidewall is whatever is left after the wheel is subtracted and halved — 8.00 in here. Expressed as a percentage of the 10.50 in section, that is 76. No metric size uses 76; the series runs in fives. The awkward number is a symptom of converting from a system that never had an aspect ratio to one that is built around it. The notation reference works through why. Load the converter for the ranked alternatives.