Ram 1500 (DT): four factory sizes inside 0.19 in, and sidewalls that vary by two inches

One height, four wheels

275/55R20 measures 31.91 in, 275/65R18 measures 32.07 in, 285/45R22 measures 32.10 in, and LT275/65R18 matches the 18-inch metric size exactly at 32.07 in. Total spread: 0.19 in, or 0.59%.

Whatever this truck was ordered with, it rolls at about 32 inches. One speedometer calibration covers the range and the trim badge changes nothing about the arithmetic.

The sidewalls are a different story:

Size	Wheel	Sidewall
275/65R18	18 in	7.04 in
275/55R20	20 in	5.95 in
285/45R22	22 in	5.05 in

Two inches of rubber separates the 18-inch fitment from the 22-inch one, on tires that stand within a fifth of an inch of each other. That is the entire trade the wheel upgrade is making.

The move with no width change at all

From 285/45R22, fitting LT285/65R20 adds 2.49 in of height and exactly zero width. Both are 285 mm casings — 11.22 in — so the change is purely vertical: 7.75% more rolling diameter, 2.24 in more sidewall, an indicated 70 mph becoming 75.4.

Single-variable changes are rare, and this one is available only from the 22-inch trim. Owners making it are usually buying sidewall rather than height, and the height is the side effect.

Three recorded ratios and where they land

3.21:1, 3.55:1 and 3.92:1 are recorded for this platform. Fitting 35x12.50R20 from 275/65R18 is a +2.93 in, 9.12% change, and the restoring arithmetic gives 3.50:1, 3.87:1 and 4.28:1 respectively — with 3.55:1, 3.91:1 and 4.27:1 the nearest sets sold.

The 3.92 case is the interesting one: the nearest ratio you can buy is 4.27:1, within a hundredth of what the arithmetic asked for, which almost never happens.

Worth noting how modest that 9.12% is in gearing terms compared with what the transmission already spans. A gearbox with several overdriven ratios responds to a change of this size by using a different one, not by running out of range.

The near-substitution

33x12.50R20 from 275/65R18 is +0.93 in and 2.88%; from 285/45R22, +0.90 in and 2.81%. Both are inside the tolerance a substitution convention would accept, which means a nominal 33 on this platform is close to a same-size change in every respect except section width — where it adds 1.67 in from the 18-inch fitment.

For a truck with coil-sprung rear suspension and, on many examples, an air system that changes ride height on its own, that width is generally the constraint that appears first.

The load side

Two of the four factory sizes here are 18-inch, and one of them is LT-marked. Between 275/65R18 and LT275/65R18 the geometry is identical to the hundredth of an inch — same diameter, same width, same 628.8 revolutions per mile — and the load rating is not. A light-truck casing reaches its rating at a much higher reference inflation than a P-metric one, which is exactly why both exist on one platform.

Substituting one for the other is invisible in every figure this site computes and highly visible on the door placard.

Why the 22-inch trim has the fewest options

Tire selection narrows sharply at 22 inches. Very few all-terrain or light-truck patterns are produced in 285/45R22, and none of the flotation sizes exist at that diameter at all — the inch-notation sizes stop at 20 on this platform.

An owner on 22s who wants a different kind of tire is therefore buying wheels as well, and the arithmetic above shows what that costs: dropping to a 20-inch wheel and 35x12.50R20 is a 9.04% increase in rolling diameter, while dropping to 33x12.50R20 is 2.81%. Two very different outcomes from what is described in conversation as the same decision.

What the arithmetic will not tell you

Whether the tire clears. Suspension configuration, wheel offset, the air suspension's ride height settings where fitted, and steering travel all decide that, and none of them appears in a size code. The air suspension is the awkward one: a truck that lowers itself at speed and again for loading has a smallest clearance it only reaches occasionally, and that is the one a tire has to survive.