275/70R18 — the three-quarter-ton size nobody calls a 33

The one size all three heavy-duty pickups share

The fourth-generation Ford F-250 Super Duty, the fourth-generation Ram 2500 and the fourth-generation Chevrolet Silverado 2500HD are recorded with the same 18-inch factory tire, and all three write it the same way: LT275/70R18.

It is already past the 33

33x12.50R18 measures 33.00 in. This size is 0.16 in taller than that — a 0.47% difference, which is nothing, but the direction is the interesting part. The tire that arrives under a 2500-class truck from the factory is not slightly short of a 33; it is fractionally over one.

The two are 1.67 in apart in section width, and that gap is the whole reason the flotation size reads as the bigger tire. It is not. A 33x12.50R18 fitted in place of this one would make an indicated 70 mph an actual 69.7 mph — a speedometer that reads very slightly high after what most people would describe as going up a size.

The three trucks list the same three upgrades, and none of them is an 18

This is the most striking thing in the records for these platforms. All three name exactly 35x12.50R20, 37x12.50R20 and LT295/65R20 as their common upgrades. Not one lists an 18-inch option.

The reason is arithmetic rather than fashion. On an 18-inch wheel, the sizes above this one are 285/70R18 at 33.71 in and 285/75R18 at 34.83 in, then a jump to 35x12.50R18 at 35.00 in. Those exist, but the wheel-and-tire market for heavy-duty trucks has largely settled on 20-inch wheels, and the listed upgrades follow the wheels people actually buy rather than the tires that are technically available.

From 275/70R18 to	Diameter	Change
285/70R18	33.71 in	+0.55 in (1.66%)
285/75R18	34.83 in	+1.67 in (5.05%)
35x12.50R18	35.00 in	+1.84 in (5.56%)
37x12.50R18	37.00 in	+3.84 in (11.59%)

The last row is where things become severe: at 11.59%, an indicated 70 mph is an actual 78.1 mph and a hundred indicated miles is 111.6.

Run the three upgrades those trucks actually list and the numbers land in almost the same places, because diameter does not care what wheel it is wrapped around. 35x12.50R20 is +1.84 in and 5.56% — identical to the 18-inch 35, since both stand 35.00 in. LT295/65R20 is +1.94 in and 5.85%, very slightly more, at 11.61 in of section width rather than 12.50. And 37x12.50R20 is +3.84 in and 11.59%, which is the same 11.59% as the 18-inch 37 for the same reason.

The wheel changes what the truck looks like and what the brake package can clear. It changes nothing in this column of percentages.

The mid-size truck reaching for it

One vehicle in the catalogue lists this size as an upgrade rather than a fitment: the Nissan Frontier D41. Its factory sizes are 255/70R17, 265/70R17 and 265/60R18, the tallest of which is 31.61 in, so reaching 275/70R18 is a 1.55 in step for it.

That is the same tire being a starting point on one truck and a project on another, and the difference between the two is not in the code. It is in the casing, the load index, and the wheel the truck came with.

Going down from here

275/65R18 sits 1.08 in below, at 32.07 in — five aspect points on the same 275 casing. That is also a factory size on the F-250, which means the Super Duty's own list contains both, an inch apart, on the same wheel.

Below that, 265/70R18 at 32.61 in is 0.55 in shorter and 0.39 in narrower, and 33x12.50R18 sits between the two at 33.00 in while being wider than either. Three sizes inside 0.55 in of height, spanning 10.43 in to 12.50 in of section width, is a fair summary of what the 18-inch truck band looks like at its upper end.

The limits of this arithmetic

Every figure above is derived from the size codes. Heavy-duty fitments in particular are governed by the load index and inflation pressure printed on the tire and on the vehicle's tire and loading placard, and none of that is deducible from three numbers and a letter. The load index table converts an index to pounds; the placard states the minimum. Run any two codes against each other in the comparison tool.