37x12.50R18 — a 37 on nobody's option list
The largest tire the 18-inch wheel takes, and the one no manufacturer's upgrade list mentions.
Flotation (imperial) notation · Flotation (imperial)
- Overall diameter
- 37.00 in
- 940 mm
- Section width
- 12.50 in
- 318 mm
- Sidewall height
- 9.50 in
- 241 mm
- Aspect ratio
- 76.0%
- sidewall as a share of width
- Wheel diameter
- 18 in
- the hole in the middle
- Circumference
- 116.2 in
- 2952 mm
- Revolutions per mile
- 545.1
- geometric, not measured
- Revolutions per km
- 338.7
- geometric, not measured
- Typical load index
- 108–128
- 2205–3968 lb per tire
Flotation sizes are almost always LT construction, so they share the light-truck band. Speed symbols run low because the aggressive tread patterns these sizes are sold in are rated against heat build-up, not against sustained high speed.
The load-index range shown is the band this CLASS of size is typically produced in — not a list of what is in stock for this exact size today. Load index and speed symbol are printed on the sidewall of the specific tire you are holding, and the required minimum is on the placard of the specific vehicle you are fitting. Read both.
Nearest real sizes in metric notation, same wheel
- 285/75R1834.83 in tall-2.17 in
- 285/70R1833.71 in tall-3.29 in
- 275/70R1833.16 in tall-3.84 in
Compare any two sizes, including ones this site has no page for, in the comparison tool.
Nothing in the vehicle records here reaches for 37x12.50R18. The 17-inch and 20-inch 37s both appear on option lists — the first on three off-road platforms, the second on three heavy-duty pickups — and this one sits between them with no recorded home.
The four 37s, compared by shape
| Size | Wheel | Sidewall | Section width | Derived aspect |
|---|---|---|---|---|
| 37x12.50R17 | 17 in | 10.00 in | 12.50 in | 80 |
| 37x13.50R17 | 17 in | 10.00 in | 13.50 in | 74 |
| 37x12.50R18 | 18 in | 9.50 in | 12.50 in | 76 |
| 37x12.50R20 | 20 in | 8.50 in | 12.50 in | 68 |
Every one of them stands 37.00 in, turns 545.1 times in a mile and covers 116.2 in per revolution. Fit any in place of any other and no instrument on the vehicle registers a change.
What separates them is the last two columns, and neither is printed on the tire. This size sits between the two 17-inch entries and the 20-inch one in casing depth, which is what an 18-inch wheel buys and costs at this diameter.
Why the gap in the middle of the range
The 17-inch 37s belong to short-wheelbase off-road platforms that use the smallest wheel their brakes allow. The 20-inch one belongs to heavy-duty pickups that leave the factory on 20s. Eighteen-inch wheels sit between those two markets and are the standard fitment for neither, so a 37 built for one is a size looking for a vehicle.
That is a statement about which platforms this catalogue records rather than about the tire.
What it would replace on an 18-inch wheel
Coming from 275/65R18 at 32.07 in the change is 4.93 in — 15.36% — which takes an indicated 70 mph to an actual 80.7 mph and a hundred indicated miles to 115.4. From 33x12.50R18 it is 4.00 in and 12.12%: 78.5 mph and 112.1 miles.
Changes of that scale move the driveline, not just the dial. Revolutions per mile drop from 628.8 to 545.1 coming from the metric size, and the axle ratio basics reference works through what a ratio would have to do to restore the original engine speed.
Related
Class: Flotation (imperial). Every figure above is computed from the size code, not stored — see how these numbers are calculated.