How tall is a 37 inch tire, really?

The only call size that is exactly what it says

37x12.50R17, 37x13.50R17, 37x12.50R18 and 37x12.50R20 all measure 37.00 in. Not approximately: the first number of a flotation code is the diameter, and above 35 in there are no metric sizes in circulation to muddy the average. Where the 31, 33 and 35 labels each cover a range of real diameters, this one covers a single value.

Sometimes the interesting fact about a number is that there is nothing interesting about it. This is one of those.

What varies instead

Since the height is fixed, the differences between these sizes live everywhere else:

• Section width — 12.50 in on most, 13.50 in on the 37x13.50R17.
• Sidewall — 10.00 in on the 17-inch sizes, 9.50 in on the 18, 8.50 in on the 20.
• Rotation count — 545.1 revolutions per mile, identical across all of them, because that follows from diameter alone.

Why there is no metric equivalent

Written in metric, a 37x12.50R17 would be a 318/80R17: a 318 mm section on an 80-series sidewall. Neither figure exists in production. Where the metric range actually gives out on a 17-inch wheel is 315/70R17, at 34.36 in — 2.64 in short of the mark, which is not a near miss but a different tire. Metric sizing stops well before this point, so the label has no competing definition to drift toward.

What comes next above it

The flotation ladder does not continue in twos past this point. Above 37.00 in the catalogue thins to a 38x13.50R20 at 38.00 in and a 40x13.50R17 at 40.00 in, with the intervening sizes rare enough that most people step straight from 37s to 40s — a jump of 3.00 in, half of which lands at the axle.

The consequence for a fitment claim

"37s" is the one call size you can take literally. Anyone quoting a clearance measurement, an arch modification or a gearing figure against 37s is quoting it against 37.00 in. Set a comparison against your current size and the difference needs no allowance for label drift on one side of it.