255/35R20 — 20-inch wheel matching a 16-inch crossover tire

Performance saloons, coupes and sporting crossovers with large factory wheels, and a substantial aftermarket population.

Four wheel sizes apart, and the same height

215/65R16 measures 27.00 in. This size measures 27.03 in. The gap is 0.09%.

One is a crossover tire on a 16-inch steel or alloy wheel with 5.50 in of sidewall. The other is a performance tire on a 20-inch wheel with 3.51 in. There is a foot of difference in the circumference of the wheels involved and none at all in how far the tires roll per revolution.

Four inches of wheel diameter, absorbed entirely by the sidewall, on two codes that share nothing else — this is about as far as the plus-sizing relationship stretches before the sidewall runs out.

The 20-inch performance sizes

Size	Diameter	Section	Sidewall
245/35R20	26.75 in	9.65 in	3.38 in
255/35R20	27.03 in	10.04 in	3.51 in
305/30R20	27.20 in	12.01 in	3.60 in
275/35R20	27.58 in	10.83 in	3.79 in
285/35R20	27.85 in	11.22 in	3.93 in

Ordered by height, the 30-series 305 lands out of width order — it is the widest of the five and the third tallest, because thirty points of aspect ratio on a very wide casing produces less sidewall than thirty-five points on a narrower one. Aspect ratio is a proportion, and proportions of different numbers are not comparable.

The three 255 mm entries, and a fourth match

255/40R19 measures 27.03 in and 255/45R18 measures 27.04 in. This size, those two, and the 215/65R16 above all agree to within 0.03 in. The three 255 mm entries are the same casing width on three wheels, spaced by exactly the amount the aspect ratio compensates for.

On this wheel, only the width is really moving

Read the sidewall column of the table above on its own. Across all five 20-inch performance sizes it runs from 3.38 in to 3.93 in — a total range of 0.55 in, or roughly the thickness of two fingers, spread over section widths that differ by 2.36 in.

That is the defining property of the wheel diameter rather than of any one size on it. Once a 20-inch rim is inside the tire, there is only so much room left, and the aspect ratios that fit in it are all shallow enough that changing them barely moves the casing depth.

So a buyer choosing among these five is choosing width, with a small amount of height attached and almost no choice about sidewall at all. On a 16-inch wheel the same five section widths would give a range of casing depths several times larger.