35x12.50R20 vs 285/65R20 — how much of a 35 survives on a 20

285/65R20 is one of the better metric approximations of a 35 anywhere in the catalogue. It arrives 0.41 in short, which on a tire nearly three feet tall is 1.18%.

The comparison runs downhill

Unusually for this site, the second size is the smaller one. A speedometer calibrated for the flotation 35 would read slightly high on the metric size, and the odometer would over-count by a little over one mile in a hundred. Every other comparison on this site runs the other way, so the sign is worth noticing before the size of it is.

Both figures are trivial in isolation. They are worth stating because the direction is the opposite of what most people assume when they are told a metric size is "the equivalent" of a flotation one — substitutions in this family more often land short than long, and this one lands short by less than most.

What a 20-inch wheel leaves behind

Sidewall, and not much of it. 7.50 in on the flotation size, 7.29 in on the metric size. Both are short for tires of this height, and the reason is arithmetic rather than design: the wheel occupies twenty of the available inches before the tire starts.

For scale, a 35x12.50R17 carries 9.00 in of sidewall — a full inch and a half more than the 20-inch version of the same nominal size. Anyone choosing 20-inch wheels for a 35-inch tire is choosing a much stiffer object than the name implies, and the derived aspect ratio reflects it: 60 for the flotation tire on a 20, against 72 on a 17.

Width is the real gap

12.50 in against 11.22 in — 1.28 in, or 10.24%. That is three times the height difference in absolute inches and around nine times as a proportion, which makes this a width comparison wearing a height comparison's clothes.

If the reason for the question is stance and arch fill, the metric size is meaningfully narrower and that is the answer. If the reason is ride height, the two are within half an inch and the question barely has an answer.

Where each one comes from

The flotation size is aftermarket by nature: sold as a package with wheels, and the standard "35s on 20s" specification. The metric size is factory-adjacent — 285/65R20 appears as an original fitment on full-size trucks and is therefore available in highway and all-terrain patterns from the same manufacturers who supply the assembly lines.

That difference in origin drives availability in opposite directions. Aggressive tread patterns skew strongly to the flotation size; quieter, longer-wearing patterns skew to the metric one. A great deal of argument about these two sizes is really about tread, with the geometry recruited afterwards.

What the numbers barely move

Revolutions per mile: 576.2 against 583.1, about seven turns per mile apart. Ride height at the axle: 0.21 in. Engine speed at a fixed road speed: a shade over one per cent different. There is nothing in this comparison that requires an allowance to be made.

Where 285/65R20 sits in its own family

The metric size is the top of a short ladder that full-size trucks are sold on: 275/55R20 at 31.91 in, 275/60R20 at 32.99 in, and 285/65R20 at 34.59 in. Each step is roughly an inch, and the last one lands within half an inch of a flotation 35. That progression is why the metric size gets substituted for a 35 so often — it is the only factory-adjacent option on a 20-inch wheel that gets close.

The step that does move things

For the same 20-inch wheel with something genuinely different underneath it, 275/60R20 against 35x12.50R20 is the version with a real diameter change in it — 2.01 in — and it is the comparison most owners of these trucks are actually making.

The part that is not geometry

Load rating and wheel fitment. An LT-marked flotation tire and a P-metric or hard-metric 285/65R20 can be hundreds of pounds apart in capacity per corner at the same nominal size. And the wheel has a second dimension that no size code states: 12.50 in of casing is specified against a wider reference rim than 11.22 in is, but the actual approved range belongs to the individual tire's data sheet and the actual width belongs to the wheel in your garage. Neither figure exists in a dimension table. The door-jamb placard is where the load requirement is stated.