Chevrolet Silverado 1500 4th gen: five factory sizes and three axle ratios

Where the trims start

Five factory sizes are recorded, and they do not cluster:

• 255/70R17 — 31.06 in
• 265/65R18 — 31.56 in
• 265/70R17 — 31.61 in
• LT275/65R18 — 32.07 in
• 275/60R20 — 32.99 in

From the shortest to the tallest is 1.94 in, or 6.24%. Two Silverados of the same model year, one on the 17-inch work-specification tire and one on the 20-inch fitment, differ by more than most owners' entire upgrade.

Note the two 18-inch entries. 265/65R18 and LT275/65R18 sit on the same wheel and differ by 0.51 in of diameter, 0.39 in of width and — the part the geometry does not show — an entirely different casing standard, with the light-truck tire rated at a much higher reference inflation.

Three axle ratios, three answers

3.23:1, 3.42:1 and 3.73:1 are recorded. Fitting LT285/65R20 at 34.59 in from the 17-inch 265/70R17 is a +2.98 in, 9.43% change, and what it asks for depends on which axle is under the truck:

Factory ratio	Ratio restoring engine speed	Nearest ratio sold
3.23:1	3.53:1	3.55:1
3.42:1	3.74:1	3.73:1
3.73:1	4.08:1	4.10:1

All three land within a few hundredths of a catalogue ratio, which is unusual and convenient. The middle row is effectively exact.

The biggest step comes from the smallest tire

35x12.50R20 from 255/70R17 is +3.94 in and 12.70% — an indicated 70 mph is really 78.9, and 100 miles of odometer is 112.7 covered. From the 20-inch 275/60R20 the same tire is +2.01 in and 6.09%, less than half the proportional change.

The pattern holds across every candidate size: the work-specification truck on the smallest wheels is making the largest change, and it is usually the truck with the least suspension clearance to absorb it.

Width is doing more of the work than height

Compare two upgrades from 255/70R17:

• 285/70R17 adds 1.65 in of height and 1.18 in of width.
• 33x12.50R20 adds 1.94 in of height and 2.46 in of width.

The flotation size adds more across than it does up: 2.46 in against 1.94 in. That inversion is the whole point, because width and height run out against different things and the width figure is the one nobody quotes when they say they are "going to 33s". Where the extra casing sits relative to the frame and the liners depends on the wheel's offset, which is not part of any size code and is not recorded here, so this page can give the 2.46 in and no more.

The 20-inch trim inverts the usual advice

Because 275/60R20 is the tallest factory fitment at 32.99 in, several sizes that read like upgrades are not. 285/70R17 is 0.28 in shorter than it. Anyone moving from the 20-inch trim to a 17-inch wheel and a nominal 33 in metric notation is reducing rolling diameter slightly and would find the speedometer beginning to over-read.

The general lesson is worth carrying to any platform with a wide trim range: an upgrade is defined against the tire you actually have, not against the smallest one the model was sold with. Two trucks in the same car park can make opposite changes by fitting the same tire.

The record has a gap at 22 inches

The factory wheel diameters recorded for this platform include a 22-inch option, and no 22-inch tire size sits against it in the five above. A sixth row would have printed in the same type, at the same width, with the same air of settlement as the five that were checked — which is why there are five.

The placard on your own truck is specific to your truck in a way that no model-level table can be, including this one.

What is geometry and what is not

Everything above is computed from the size codes and is exact. Whether a tire clears the wheel well, the liner and the steering components on your Silverado depends on its cab, its suspension, its wheel offset and what has already been changed. The sizes recorded as common upgrades are observations of what owners fit.