How wide is a 285 tire?

11.22 inches, and nothing else

The leading number of a metric code is a length in millimetres and it is the only part of the size that means anything on its own. 285 mm converts to 11.22 in, and that figure is identical whether the code continues 285/70R17, 285/75R16 or 285/65R20.

The same width on three quite different tires

Those three sizes measure 32.71 in, 32.83 in and 34.59 in tall respectively. Nearly two inches of diameter separates the shortest from the tallest, and all three are "a 285". This is the trap that makes width-only shorthand useless in a fitment discussion: someone saying they run 285s has told you what their tire measures across, and nothing about what it measures around.

Where 285 sits between the two notations

11.22 in falls awkwardly between the flotation widths, which are sold in half-inch steps: it is wider than a 10.50 and narrower than an 11.50, closer to the latter. That is why a 285 metric size and a 12.50 in flotation size are so often discussed as alternatives when they are 1.28 in apart — put them side by side and the width gap is larger than the height gap by more than four to one.

What is not 11.22 inches

Two things, both frequently assumed. The tread is narrower — the casing bulges outboard of the ribs, so the rubber actually touching the road is well inside the 285 figure, and by an amount that varies with tread pattern rather than with size code. And the mounted measurement moves with rim width, since the quoted figure applies at one specified measuring rim; the rim width to tire width page has that relationship, and section width against tread width has the other.

What the 285 does describe precisely is the widest point of the casing under standard conditions, which is the dimension a wheel arch and a suspension arm care about — and the reason it is the number quoted on the sidewall rather than the tread figure anyone would find more intuitive.