Ram 2500 4th gen: the 17-inch fitment is the outlier

Two clusters, not three sizes

LT275/70R18 at 33.16 in and LT285/60R20 at 33.46 in are 0.31 in apart — under 1%. Then LT265/70R17 at 31.61 in sits 1.86 in below the taller of them, a 5.88% gap.

So the platform effectively has two starting points: a little over 33 inches on the 18 and 20-inch wheels, and a little over 31.5 on the 17. Every figure below splits along that line.

All three are LT-marked, which is expected on a three-quarter-ton truck and which means the load index behind the placard pressure is doing real work. A substitution that clears geometrically can still fall short of what the vehicle was certified with, and the placard is where that requirement is written.

What 35s and 37s cost from each cluster

35x12.50R20 at 35.00 in:

• From LT285/60R20: +1.54 in, 4.59%.
• From LT275/70R18: +1.84 in, 5.56%.
• From LT265/70R17: +3.39 in, 10.74% — more than double the proportional change.

37x12.50R20 at 37.00 in, exactly two inches above the 35 in absolute terms:

• From LT285/60R20: +3.54 in, 10.56%.
• From LT275/70R18: +3.84 in, 11.59%.
• From LT265/70R17: +5.39 in, 17.07% — an indicated 70 mph becomes 81.9, and 100 miles of odometer is 117.1 covered.

The pattern is worth stating plainly: from the 20-inch fitment, a 37 is a smaller proportional change than a 35 is from the 17-inch fitment.

The narrow 35

LT295/65R20 measures 35.10 in — a tenth of an inch taller than the flotation 35 — on 11.61 in of section width against 12.50 in. Nearly an inch narrower for the same height, in a light-truck casing.

From LT285/60R20 that is +1.63 in and 4.88%, adding just 0.39 in of width. On a heavy-duty truck with a solid front axle, where the steering sweeps the tire through a wide arc against the frame and the radius arms, that width difference is frequently the deciding factor rather than the height.

Gearing matters more when the truck is loaded

The ratio column is empty here, which on a three-quarter-ton is the least convenient field to be missing and the hardest to generalise: the same truck was sold with a gasoline V8 and a diesel six, and those two were not geared alike.

What holds regardless: a 17.07% increase in rolling diameter reduces torque at the road by 17.07% and takes the same fraction off engine speed at any given road speed. On a truck bought to pull weight, that is the change that will be noticed first, before the speedometer error and long before the appearance.

Your own ratio, entered into the gear ratio calculator alongside the two sizes, gives the figure that would restore engine speed and the nearest ratio sold as a set.

Two inches of tire is one inch of axle

The gap between the 35 and the 37 is exactly 2.00 in of diameter, and it is worth being precise about what that buys. Half of it goes upward from the axle centreline toward the fender, and half of it goes downward to the ground: the axle sits 1.00 in higher, and the differential housing — the lowest fixed point under most of these trucks — rises by the same inch.

Ground clearance at the axle is therefore half the number in the tire's name, while the clearance problem at the fender is the full number. That asymmetry is why a two-inch tire step so often costs more in bodywork than it returns in usable clearance.

The load question sits alongside the geometry

Every figure above is exact geometry. None of it addresses whether a given tire's load index meets the requirement on your placard at the pressures you run, or whether it clears the steering components at full lock with the suspension compressed. On this class of truck both questions deserve more attention than the diameter does, and neither is answerable from a size code.