255/70R17 — the mid-size truck size, and a very exact one

The size, and the reason its arithmetic is unusually tidy

Overall diameter 31.06 in — 789 mm. Section width 10.04 in. Sidewall 7.03 in. Circumference 97.6 in and 649.4 revolutions per mile.

At 255 mm of section width, five points of aspect ratio are worth 2 × 0.05 × 255 mm of diameter, which is 25.5 mm, which is 1.004 in. The metric-to-imperial conversion very nearly cancels, and the result is that the 255 mm family steps in almost exact whole inches:

Size	Overall diameter
255/65R17	30.05 in
255/70R17	31.06 in
255/75R17	32.06 in
255/80R17	33.06 in

One inch per five points, four rungs, on one wheel. No other section width in this catalogue lines up that neatly, and it makes 255 mm the easiest width to reason about without a calculator.

Fitment

This is the common upper fitment on the mid-size body-on-frame class: 4Runner, Tacoma in its off-road trims, Colorado and Canyon, and a number of full-size half-tons in base specification. Those vehicles generally leave the factory somewhere between 30 and 31.5 in of rolling diameter, and this size sits comfortably inside that band while being wider than most of what shares it.

Not a 32

At the half-inch granularity people speak in, this is a 31. It measures 31.06 in, and it is routinely described as a 32 by people who have seen it mounted next to something smaller.

The genuine 32 on this wheel is 32x11.50R17, which measures exactly 32.00 in because in flotation notation the diameter is the printed number. That is 0.94 in taller than this size and 3.04% up, with 1.46 in more section width. The gap is real and the appearance gap is larger than the diameter gap, because the flotation size is the wider tire by a considerable margin.

Ten-millimetre steps, and the crowded band around 31 inches

At the same aspect ratio and wheel, 245/70R17 is 0.55 in shorter at 30.50 in and 265/70R17 is 0.55 in taller at 31.61 in. Around this diameter the 17-inch catalogue is dense: 245/75R17 lands at 31.47 in, 265/65R17 at 30.56 in, 255/65R17 at 30.05 in.

Six sizes on one wheel within about 1.6 in of each other is not an accident of listing. It reflects how many different vehicles target the same rolling diameter for the same drivetrain reasons and arrive at it from different width and sidewall preferences.

Its counterpart one wheel size up

Two 18-inch sizes in the same 255 mm family are worth putting beside this one.

255/65R18 measures 31.05 in against this size's 31.06 in — a difference of 0.01%, which is as close to identical as two different codes get. An inch of extra wheel, five points less aspect ratio, and the same rolling diameter: the plus-one relationship performed exactly. The sidewall falls from 7.03 in to 6.53 in.

255/70R18 keeps the aspect ratio and takes the extra inch of wheel, so it stands exactly 1.00 in taller at 32.06 in — 3.22% up, half an inch of it at the axle. Same width, same sidewall depth, one inch more everything else.

The most versatile width in the catalogue

255 mm appears more times here than any other section width — fifteen entries, from 255/35R19 at 26.03 in to 255/80R17 at 33.06 in. That is seven inches of diameter range on a single tread stance, covering low-profile performance fitments, crossovers, and light-truck sizes with no change to the first three digits of the code.

It is a useful reminder that the width figure alone predicts nothing about the tire's height, and the tire size code reference walks through why.

Load, and the two constructions this size comes in

The panel above gives a class-level load index range with the method stated. In this size that range is genuinely wide, because the code is sold both as a passenger-standard casing for crossovers and as an LT-marked light-truck casing for the same vehicles used differently. The prefix changes the reference inflation and therefore the rating, not the geometry — an LT-marked 255/70R17 is still 31.06 in tall.

Two indexes, one arithmetic step: the load index table turns either of them into pounds.

Why the 649.4 is a starting point

A tire's rolling radius under a vehicle is smaller than the radius the code describes, and the gap grows with how much casing there is to deflect. This size has 7.03 in of it, which is why the overall diameter and rolling radius reference matters more here than it would on a low-profile fitment.

There is a second, opposite effect specific to this part of the market. Aggressive light-truck tread patterns are moulded on shoulder profiles that commonly come in under the nominal diameter when new, so the same tire can be short of 31.06 in unloaded and shorter still loaded. The two effects push the same way and neither is in the code.