305/70R17 — 33.81 in, in the band nobody counts

The rungs the shorthand leaves out

Ask what size someone is running and the answer usually comes back as an even number: 31s, 33s, 35s, 37s. That habit comes from flotation notation, where the first number is the diameter, and the sizes that were mass-produced happened to be spaced two inches apart.

Metric codes do not respect that spacing. On a 17-inch wheel, this catalogue holds five sizes between a flotation 33 and a flotation 35: 255/80R17 at 33.06 in, 295/70R17 at 33.26 in, 305/70R17 at 33.81 in, 34x10.50R17 at 34.00 in and 315/70R17 at 34.36 in. Three of the five — including this one — appear on no vehicle record here as either a factory fitment or a listed upgrade.

They are not rare because they are bad. They are rare because the conversation has no word for them.

Almost a 35 across, well short of a 35 in height

	33x12.50R17	305/70R17	35x12.50R17
Diameter	33.00 in	33.81 in	35.00 in
Section width	12.50 in	12.01 in	12.50 in
Sidewall	8.00 in	8.41 in	9.00 in

Read the middle column against either side. This size is 0.81 in taller than a 33 and 1.19 in shorter than a 35, but its section width is within half an inch of both — 0.49 in narrower, in each case, than the flotation size it is being compared with.

So it is not a compromise between the two in the way its diameter suggests. In width it is essentially a 12.50 that measures a little under; in height it is closer to the 33 than to the 35.

The other way to occupy the 34-inch band

34x10.50R17 stands 34.00 in — 0.19 in taller than this size — on 10.50 in of section width. That is 1.51 in narrower.

Two sizes, a fifth of an inch apart in height, an inch and a half apart in width, and both essentially invisible in the fitment records. They represent opposite answers to the same question: 305/70R17 keeps the width of a big tire and gives up height, while 34x10.50R17 keeps a round number of inches and gives up width.

Arriving here from below

Coming from	Diameter	Step	Percentage
285/70R17	32.71 in	+1.10 in	3.37%
33x12.50R17	33.00 in	+0.81 in	2.46%
295/70R17	33.26 in	+0.55 in	1.66%

The middle row is the one that would surprise most people: a set of 33s replaced by these gains less than an inch of height and loses half an inch of section width.

Going the other way, 315/70R17 is 0.55 in taller and 0.39 in wider — the same ten-millimetre step in section width that separates most adjacent metric sizes, and the reason the two are frequently cross-shopped. 35x12.50R17 is 1.19 in taller at 3.52%, which turns an indicated 70 mph into an actual 72.5 mph. 37x12.50R17 is 3.19 in taller at 9.43%, and 76.6 mph.

The odd shape of a 70-series 305

A 305 mm casing at 70% aspect produces 8.41 in of sidewall — within a hundredth of the sidewall on a 285/75R16, which is a very different tire on a very different wheel.

That is the recurring oddity of high-aspect wide sizes: the percentage is applied to a large number, so a shape that reads as conventional on paper produces a lot of sidewall in absolute terms. The 33x12.50R17 it is most often compared with has 8.00 in, and the 35x12.50R17 above it has 9.00 in, so on sidewall alone this size sits almost exactly between them — closer to the midpoint than its diameter is.

The catalogue's other 305

305/55R20 is the same 12.01 in of casing, fifteen aspect points lower, on three more inches of wheel. It stands 33.21 in against this size's 33.81 in — 0.60 in shorter, 1.78% — and it does it on 6.60 in of sidewall rather than 8.41 in.

The pair is a clean demonstration of what the middle number does when the first one is held fixed. The width does not move at all. The height moves by less than two-thirds of an inch. The sidewall moves by nearly two inches, all of it transferred into wheel. Revolutions per mile go from 596.5 here to 607.3 there — under two per cent, on tires that look nothing like each other.

Off a 17-inch wheel entirely, the closest thing in the catalogue is 285/70R18 at 33.71 in: a tenth of an inch shorter, 0.79 in narrower, and the size that a buyer looking at this one has usually been offered instead, because the 18-inch wheel has fitments behind it and the 17-inch version of this diameter does not.