Ford F-250 Super Duty 4th gen: every factory size is a light-truck tire

Four sizes, all LT

LT245/75R17 at 31.47 in, LT275/65R18 at 32.07 in, LT275/70R18 at 33.16 in and LT285/60R20 at 33.46 in. The spread from the shortest to the tallest is 2.00 in, or 6.34%.

The prefix is the important part. An LT tire is built and rated to the light-truck standard, with a much higher reference inflation behind its load index, and on a three-quarter-ton truck the load figure on the placard is not decorative. A passenger-construction tire in the same nominal size is a different object with a different rating, whatever the geometry says.

Everything this site computes — diameter, width, sidewall, revolutions per mile — is identical between a P-metric and an LT tire of the same size. The number that is not identical is the one that matters most on this platform.

The 18-inch pair is worth separating

LT275/65R18 and LT275/70R18 sit on the same wheel and the same 10.83 in section width. Five points of aspect ratio separate them, worth 1.08 in of diameter — 3.38% — and 0.54 in of sidewall.

That is a genuine trim-level difference hiding behind two nearly identical size codes, and it is the kind of thing worth checking on the placard rather than assuming from the wheel.

The step to 35s and 37s

35x12.50R20 at 35.00 in:

• From LT285/60R20: +1.54 in, 4.59%. Almost a substitution.
• From LT275/70R18: +1.84 in, 5.56%.
• From LT275/65R18: +2.93 in, 9.12%.
• From LT245/75R17: +3.53 in, 11.22%.

37x12.50R20 at 37.00 in is exactly two inches taller than the 35 in every case, which changes the percentages considerably: +3.54 in and 10.56% from the 20-inch fitment, and +5.53 in and 17.58% from the 17-inch one. At that figure an indicated 70 mph is 82.3 and 100 odometer miles is 117.6 travelled.

LT295/65R20 at 35.10 in reaches the same height as the flotation 35 with 0.89 in less section width, which on a solid-front-axle truck with a large turning radius is the trade worth thinking about.

The tallest factory tire has the least sidewall

Read the four sizes by sidewall rather than diameter and the ranking inverts. LT275/70R18 carries 7.58 in, LT245/75R17 carries 7.23 in, LT275/65R18 carries 7.04 in — and LT285/60R20, the tallest tire of the four at 33.46 in, carries only 6.73 in.

The 20-inch package reaches its height through the wheel rather than through the tire, which is a different proposition on a truck that spends time on unpaved surfaces. Less sidewall means less deflection available before the rim takes the load, and it is the reason so many owners of that package move down a wheel size when they change tires.

Gearing on a truck that is expected to pull

The ratio column is blank, and on this truck of all trucks that is the field a reader most wants. It is also the field most likely to differ between two examples parked next to each other, because a three-quarter-ton is specified around what it will pull.

The relationship holds anyway, and it is more consequential here than on a half-ton: a 17.58% diameter increase reduces torque at the road by the same 17.58% and lowers engine speed at a given road speed by the same fraction. On a truck rated to tow at weights where the gearing was chosen carefully, that is not a cosmetic change, and the arithmetic is worth doing before the tires are ordered rather than after.

The gear ratio calculator will take your truck's own ratio and return the figure that restores the original engine speed.

What the numbers cannot cover

Whether the tire clears, and whether its load rating meets what the placard requires at the pressures your truck is set up for. On a three-quarter-ton the second question is the harder one: the placard here can ask for a Load Range E casing at 80 psi, and a flotation size that clears the arches easily may not be produced in a casing that carries the rear axle rating. Clearance is solved with a tape measure; capacity is solved by reading the sidewall against the placard.