275/60R20 — a 33-inch tire that does not say so

Five trucks already leave the factory on this

Five vehicle records name 275/60R20 as factory equipment — four full-size pickups from three manufacturers and one full-size SUV, all on a 20-inch wheel. The panel above has them. In the only sense the arithmetic recognises, every one of those trucks is delivered on 33-inch tires.

Two codes, one height, one sidewall

Set it against 33x12.50R20 and the correspondence is closer than the "is a 285 really a 33" argument ever gets:

	275/60R20	33x12.50R20
Overall diameter	32.99 in	33.00 in
Sidewall height	6.50 in	6.50 in
Section width	10.83 in	12.50 in
Aspect ratio	60	52
Revolutions per mile	611.3	611.2

The height difference is 0.01 in — 0.02%, or one part in five thousand. At an indicated 70 mph the actual speed is 70.0 mph. Over 100 indicated miles the true distance is 100.0. There is no measurable consequence of any kind, and a real tire's mould tolerance is an order of magnitude larger than the gap.

The sidewalls match to the hundredth as well: 275 mm at 60% is 165 mm, which is 6.50 in, and a 33 on a 20-inch wheel has 13 in of tire above and below the rim, which is also 6.50 in per side.

What is actually different, and why the aspect ratios disagree

One dimension separates them: 1.67 in of section width.

That single difference is also why the two codes report different aspect ratios for tires with identical sidewalls. Aspect ratio is sidewall expressed as a percentage of section width, so the same 6.50 in of sidewall is 60% of a 275 mm casing and 52% of a 12.50 in one. The 33's aspect ratio is not printed anywhere on it, and if it were, the number would look like a low-profile tire's.

This is the clearest available demonstration that aspect ratio describes a shape, not a height. Two tires can share a sidewall to the hundredth of an inch and be eight aspect points apart.

Every one of those five trucks lists the flotation 33 as an upgrade

That is not an exaggeration of the records: the F-150, Silverado, Sierra, Ram and Tahoe all name 33x12.50R20 among their common upgrades while shipping 275/60R20 from the factory.

The step is 0.01 in of height and 1.67 in of width. Nothing about the speedometer, the odometer, the axle ratio or the engine speed at a given road speed changes. What changes is section width, which is the dimension none of the arithmetic on this site can see and the only one that is actually moving.

The steps that do change something

Target	Diameter	Change	Speed at an indicated 70 mph
275/55R20	31.91 in	−1.08 in (3.28%)	67.7 mph
285/60R20	33.46 in	+0.47 in	71.0 mph
275/65R20	34.07 in	+1.08 in (3.28%)	72.3 mph
285/65R20	34.59 in	+1.59 in (4.83%)	73.4 mph
35x12.50R20	35.00 in	+2.01 in	—

Note the symmetry in rows one and three: five aspect points down and five aspect points up are both 1.08 in on a 275 casing, for the same reason five points on a 265 is always 1.04 in. The percentage is applied to a section width that is not changing.

One more sits between the rows and is easy to miss: 305/55R20 at 33.21 in. It is only 0.22 in taller than this size, and 1.18 in wider — a third route to a wider tire that costs almost nothing in diameter, and therefore nothing in speedometer error either. Where 33x12.50R20 adds width at 0.01 in of height, this one adds slightly less width for slightly more height.

How long it takes for a hundredth of an inch to matter

It is worth putting a number on "no measurable consequence", because 0.01 in is not zero and the error does accumulate.

Drive 10,000 indicated miles on 33x12.50R20 after leaving the factory on this size and the true distance is 10,002 miles. Drive 100,000 and it is 100,024. Twenty-four miles, over a distance most of these trucks will not reach.

Set that against the same trucks' other listed upgrade. 285/65R20 is 4.83% taller, and 100,000 indicated miles on it is 104,833 — a discrepancy of nearly five thousand miles, arriving from a step that looks, in the option list, like the same kind of decision as the flotation 33.

Downward, 265/60R20 is 0.47 in shorter at 1.43%. That is the smallest real change available on this wheel, and it is still large enough to see on a trip meter over a single tank.