What is 265/70R17 in inches?

Called a 31 by some people and a 32 by others

31.61 in is an unsayable number, so almost nobody says it. Round it down and you have "31s"; round it up and you have "32s"; both camps are describing this same casing. That is worth knowing before you read anyone's account of what came off their truck, because the label they used is a preference, not a measurement.

More than half of the height is wheel

Take the 17 in wheel out of the overall diameter and 14.61 in of rubber remains, split evenly between the sidewall above the wheel and the sidewall below it — 7.30 in at each end. The part of this tire that can flex, take a pothole or be aired down is that 7.30 in, and everything else in the 31.61 is rim.

Two codes, one height

265/75R16 stands 31.65 in: four hundredths of an inch taller, on a wheel a full inch smaller, at exactly the same 10.43 in section width. For anyone moving between a 16-inch and a 17-inch wheel while leaving the instrument cluster alone, that is as close a pair as this catalogue holds. Draw the two together and the whole difference turns out to be sidewall — 7.30 in against 7.82 in — with nothing left over at the top of the tire.

638.1 revolutions per mile is the figure that matters downstream

A speedometer calibrated for this size is counting 638.1 wheel rotations to the mile. Fit anything taller and the count falls, the needle reads low by exactly the proportion the diameter grew, and the odometer drifts with it. Because 265/70R17 is the size that most often comes off a mid-size truck first, that figure is the baseline behind a great deal of the 33-inch upgrade arithmetic elsewhere on this site.

The middle number of the code is where all of this originates: 70 means the sidewall is 70% of the 265 mm section width, and the aspect ratio is the one field that changes the height of a tire without changing the width printed on it.