245/35R20 — the narrowest 35-series on a 20-inch wheel

Nothing on a 20-inch wheel in this catalogue is narrower. The size appears on performance saloons and coupes with the largest factory wheel option, and widely in the aftermarket on cars whose arches will not take a wider tire at that diameter.

The 35-series on a 20

Size	Section	Diameter	Height added
245/35R20	9.65 in	26.75 in	—
255/35R20	10.04 in	27.03 in	+0.28 in
275/35R20	10.83 in	27.58 in	+0.83 in
285/35R20	11.22 in	27.85 in	+1.10 in

From the narrowest to the widest is 1.57 in of section width and only 1.10 in of diameter — 4.12%. At a 35-series aspect ratio the tire grows sideways far faster than it grows upwards, which is exactly the property that makes very wide fitments possible without raising the car.

A 20-inch wheel inside a 26.75 inch tire

Of the standing height, 20 in is wheel. The rubber accounts for the rest, and 3.38 in of it sits between the rim flange and the road at each end.

That is the least sidewall of any size on this page's wheel diameter, and it is the practical limit of the plus-sizing trade: a further inch of wheel at this section width would leave under three inches of casing.

Its counterparts on smaller wheels

245/40R19 measures 26.72 in and 245/45R18 measures 26.68 in. All three carry the same 9.65 in section and stand within 0.07 in of each other, so any of them can be fitted where the others were without touching the speedometer. Sidewall runs 3.38, 3.86 and 4.34 in across the three.

225/55R17 lands 0.01 in away at 26.74 in, from a completely different direction — narrower tread, much deeper sidewall, three inches less wheel.

Above the performance band there is a gap

The five performance sizes on a 20-inch wheel run from this one at 26.75 in to 285/35R20 at 27.85 in. The next catalogued 20-inch size above them is 265/50R20 at 30.43 in.

That is a 2.58 in hole with nothing in it — more than the entire performance group's own spread, which is 1.10 in. Nothing is produced there because nothing wants it: below 28 in the 20-inch wheel is serving cars, above 30 in it is serving trucks and large SUVs, and no vehicle asks for a tire that splits the difference.

It is the sharpest illustration on any wheel diameter that a catalogue of real sizes is not a grid. The arithmetic would happily produce a 245/45R20 or a 265/40R20; the market has no use for either, so neither is here.