GMC Sierra 1500 4th gen: width matters more than height

The four recorded sizes

255/70R17 at 31.06 in, 265/65R18 at 31.56 in, LT275/65R18 at 32.07 in and 275/60R20 at 32.99 in. From shortest to tallest is 1.94 in, or 6.24%.

Two of the five share an 18-inch wheel, and the pairing repays a second look. Between them lie 0.51 in of height and 0.39 in of width, which is small; and a change of casing standard, which is not. The LT prefix moves the tire onto the light-truck basis, where the load index is quoted against a much higher reference inflation — the same nominal jump in size, arriving with a different rulebook attached.

The width column is the one to read

Take the three sizes owners commonly step to, measured from 255/70R17, and put height and width side by side:

Fitted	Height added	Width added
285/70R17	+1.65 in	+1.18 in
33x12.50R20	+1.94 in	+2.46 in
LT285/65R20	+3.53 in	+1.18 in

The middle row adds more section width than diameter. A truck fitted with that tire has gained about an inch and a quarter on each side at the widest point of the casing, against less than an inch of extra radius at the top.

Wheel wells on a half-ton are usually more forgiving upward than inward, and the inward dimension is also where the wheel's offset gets involved. Quoting a size change purely as "plus two inches" describes the smaller of the two changes being made.

Where each starting point lands

LT285/65R20 at 34.59 in is the tallest of the common upgrades:

• From 255/70R17: +3.53 in, 11.37%. An indicated 70 mph is 78.0, and 100 odometer miles is 111.4 covered.
• From 265/65R18: +3.02 in, 9.58%.
• From LT275/65R18: +2.51 in, 7.83%.
• From 275/60R20: +1.59 in, 4.83%.

Four trucks, one tire, and the percentage more than doubles between the top and bottom of the trim range. The truck with the smallest factory tire is also, usually, the one with the least clearance to give.

The 20-inch trim's peculiar position

275/60R20 is the tallest recorded factory fitment at 32.99 in, which puts several nominal upgrades below it. 285/70R17 is 0.28 in shorter — fitting it would reduce rolling diameter by 0.86% and make the speedometer read slightly high.

An owner moving from the 20-inch trim to a 17 or 18-inch wheel package for tire choice or sidewall is not automatically making an upgrade in the arithmetic sense, whatever the tire is called.

The modest option nobody talks about

33x12.50R20 measured from 275/60R20 is a 0.01 in change. From LT275/65R18 it is +0.93 in, 2.88%. On the two upper trims, in other words, a flotation 33 is close to a straight substitution in height, and everything it changes is width and tread pattern.

That reframes the decision usefully. If the object is appearance and grip on loose surfaces, the 20-inch trims can have it without touching the speedometer, the odometer or the effective final drive. If the object is ground clearance, this size is not the one delivering it.

Two absences in the record

Two fields on this page are empty: the axle ratios, and the tire size belonging to the 22-inch wheel diameter that is recorded. Neither blank is a claim that the thing does not exist — the wheel diameter is listed precisely because it does. They are different kinds of gap, though. The missing tire size can only come from a specification sheet; the missing ratio is stamped on the truck.

For the engine-speed question, the gear ratio calculator takes your own truck's ratio directly.

The limit of what is computed here

Diameter, width, sidewall, speedometer error and revolutions per mile are exact consequences of the size codes. Whether a tire clears your Sierra's liners, sway bar and steering at full lock is not computable from any of them, and depends on the cab, the suspension and the wheel offset in front of you.