38x13.50R20 — the smallest step on the flotation ladder
An odd number in a notation that deals in even ones, and the only 38 here.
Flotation (imperial) notation · Flotation (imperial)
- Overall diameter
- 38.00 in
- 965 mm
- Section width
- 13.50 in
- 343 mm
- Sidewall height
- 9.00 in
- 229 mm
- Aspect ratio
- 66.7%
- sidewall as a share of width
- Wheel diameter
- 20 in
- the hole in the middle
- Circumference
- 119.4 in
- 3032 mm
- Revolutions per mile
- 530.7
- geometric, not measured
- Revolutions per km
- 329.8
- 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
- 325/60R2035.35 in tall-2.65 in
- 295/65R2035.10 in tall-2.90 in
- 285/65R2034.59 in tall-3.41 in
Compare any two sizes, including ones this site has no page for, in the comparison tool.
Nothing in the vehicle records here lists 38x13.50R20. It is the largest size catalogued on a 20-inch wheel and the only 38 on the site.
One inch, where the ladder usually moves two
The imperial range runs 30, 31, 32, 33, 35, 37 — single inches at the bottom and two-inch steps once past the 33. This size breaks that pattern at the top:
| Size | Diameter | Step from the rung below |
|---|---|---|
| 33x12.50R20 | 33.00 in | — |
| 35x12.50R20 | 35.00 in | +2.00 in (6.06%) |
| 37x12.50R20 | 37.00 in | +2.00 in (5.71%) |
| 38x13.50R20 | 38.00 in | +1.00 in (2.70%) |
2.70% is the smallest gap between two adjacent flotation sizes anywhere in this catalogue, and it arrives at the point where the percentages were shrinking anyway. Coming from a 37 the change is barely more than a tape-measure discrepancy: an indicated 70 mph becomes an actual 71.9 mph.
The inch of width that comes with it is the larger part of the change. 13.50 in against 12.50 in is a stance difference of the same order as the entire spread between the metric options on this wheel.
The widest thing this wheel takes
At 13.50 in of section, this size is 0.70 in wider than 325/60R20 — the widest metric code catalogued — and 1.00 in wider than every flotation 12.50 on the wheel. It is also 2.65 in taller than the 325.
Only two other codes in this catalogue reach 13.50 in, and both are on 17-inch wheels. This is the only place that width appears on a large-diameter wheel.
What it would replace
- 35x12.50R20 — 3.00 in below, 7.89%, 1.00 in narrower
- 295/65R20 — 2.90 in below, 7.64%, 1.89 in narrower
- 285/65R20 — 3.41 in below, 8.98%, 2.28 in narrower
- 305/55R20 — 4.79 in below, 12.61%, 1.49 in narrower
Every one of those is a substantial change in both dimensions at once, which is what happens at the end of a ladder: there is nothing left to step from that is close by. What a change of this size does to the relationship between engine speed and road speed is set out in the transmission gear ratios reference.
Related
Class: Flotation (imperial). Every figure above is computed from the size code, not stored — see how these numbers are calculated.