Chevrolet Silverado 1500 3rd gen: every factory size within 0.85 in

A tight factory range

Three sizes are recorded: 255/70R17 at 31.06 in, 265/65R18 at 31.56 in, and 275/55R20 at 31.91 in. Total spread 0.85 in, or 2.75% — a tighter grouping than the generation that replaced this one, and tight enough that the trim you have shifts the arithmetic only slightly.

Notice the direction: here the largest wheel carries the tallest tire, which is the opposite of what several other platforms on this site do. The 20-inch fitment is 2.75% taller than the 17-inch one rather than shorter.

The upgrade sizes are close to the factory ones

The three sizes owners commonly move to are all within an inch and a half of where they started:

Fitted	Diameter	From 255/70R17	From 275/55R20
285/70R17	32.71 in	+1.65 in (5.32%)	+0.80 in (2.50%)
33x12.50R20	33.00 in	+1.94 in (6.26%)	+1.09 in (3.42%)
LT285/60R20	33.46 in	+2.41 in (7.76%)	+1.56 in (4.87%)

From the 20-inch fitment, two of the three are inside the 3% band that substitution conventions usually work to. A 20-inch truck fitting a nominal 33 is making a change smaller than a lot of owners assume they are making.

The largest change on the list — the base 17-inch truck to LT285/60R20 — is 7.76%, which takes an indicated 70 mph to 75.4.

What LT285/60R20 is buying

It reaches 33.46 in, half an inch taller than the flotation 33, with 11.22 in of section width against the flotation tire's 12.50 in. More height, an inch and a quarter less width, and a light-truck casing rather than a passenger one.

That combination is the reason it appears in build lists on a truck whose front wheel wells are not generous. Height is usually easier to accommodate than width on this platform, and the LT construction matters on a vehicle with a payload rating.

Gearing, with the ratio column blank

Nothing above needed a ratio, because nothing above is a gearing figure.

What the geometry does say is that a 6.26% diameter increase behaves exactly like a 6.26% numerically lower final drive, whichever ratio is under your truck. With that ratio in hand — from the differential tag or the vehicle's build sticker — the gear ratio calculator will return the ratio that restores your original engine speed and the nearest one sold as a set.

Given the modest size of the changes on this platform, the arithmetic rarely points at a regear. Under 5%, the transmission's own ratio steps absorb most of it.

A 22-inch wheel with no recorded tire

The factory wheel diameters include a 22-inch option, and no 22-inch tire size is recorded among the three above. That is a gap in what could be confirmed rather than a statement that the fitment did not exist — and filling it with a plausible guess would have made the record look complete while making it less reliable.

Sidewall is what actually changes between the trims

The three factory sizes are close in height and far apart in sidewall: 7.03 in on the 17-inch fitment, 6.78 in on the 18, and 5.95 in on the 20. That is 1.07 in between the ends of the range, on tires whose overall diameters are within 0.85 in of each other.

That is the trade the trim range is actually making. The wheel grows, the sidewall shrinks to hold the rolling diameter, and what changes is how much rubber sits between the rim flange and a pothole edge — not how tall the truck stands.

The line this page does not cross

None of the above says a size fits. This truck was sold in three cab lengths against two bed lengths, in two- and four-wheel drive, with and without a factory off-road package that changes both ride height and wheel offset — and the tire that clears one of those combinations is not guaranteed to clear the next. The geometry is identical across all of them; the space around it is not.