What is 37x12.50R17 in metric?

The gap widens as the tire grows

Convert a flotation 33 to metric and the nearest real size misses by six hundredths of an inch. Convert a 35 and it misses by 0.64 in. Convert this one and it misses by 2.64 in. The pattern is not a coincidence. Metric sizing has a ceiling on this wheel and flotation sizing does not, so once past the ceiling every further step in the imperial ladder adds its whole length to the shortfall.

What 318/80R17 would have to be

An 80-series sidewall on a 318 mm section — 12.50 in of width carrying 10.00 in of sidewall at each end. Metric aspect ratios of 80 exist in narrow sizes such as 255/80R17, but not at this width, and 318 mm is not a manufactured section in any series. Both halves of the answer are unavailable independently, which is a reasonable definition of an impossible size.

Nothing metric is close

• 315/70R17 — 34.36 in. The tallest available, and still 2.64 in short.
• 305/70R17 — 33.81 in. Further again.

Against a speedometer calibrated for a genuine 37, the 315 reads 7.13% high: an indicated 70 mph is 65.0 mph. That is not a substitution, it is a different tire that happens to share a wheel size.

The width, for once, is not the problem

315/70R17 is 12.40 in of section against 12.50 in — a tenth of an inch, closer than almost any other metric-to-flotation width match on any wheel. Every other conversion on this site fails on width and succeeds on height. This one does the reverse, which makes it the clearest demonstration that the two notations are independent in both dimensions rather than systematically offset in one.

What this means in practice

Above roughly 34.5 in on a 17-inch rim, flotation notation is the only option available, and the question of a metric equivalent stops being meaningful. Anyone specifying a build at 37 in is specifying a flotation size whether they intended to or not. Load the converter to see the ranked list, and the 37-inch call size for why every one of them measures exactly 37.00 in.