What is 265/65R17 in inches?

Shorter than the number suggests

A 265 on a 17-inch wheel sounds like a 31-inch tire. This one is 30.56 in, which rounds to 30.5 and gets called a 31 anyway. The 65-series sidewall is what holds it down: 6.78 in, against the 7.30 in a 70-series gives on the same section.

The cleanest single-variable experiment in tire sizing

265/65R17 and 265/70R17 differ in exactly one character. Same width, same rim, same construction. The diameter difference is 1.04 in and the width difference is zero. If you want to know what five points of aspect ratio are worth on a 265 mm section, that is the answer, and it holds wherever the same pair of numbers appears.

On the instruments the pair is 3.41% apart, which makes this the closest real-world approximation to "one inch of tire" available on a 17-inch truck wheel — the reason it turns up as the worked example on the one-inch speedometer question.

Seen as a count rather than a length, the same gap is 659.9 revolutions per mile here against 638.1 for the 70-series: nearly twenty-two extra turns of the wheel in every mile driven. That count, not the diameter, is what the vehicle's electronics are actually working from.

A near-twin two wheel sizes up

265/60R18 measures 30.52 in: four hundredths shorter, on a wheel an inch larger, at the same 10.43 in of section. Trading an aspect point for a rim inch is an even swap here, which is exactly what the plus-size ladders on crossovers rely on.

What it is fitted to

This is a crossover and light-SUV size rather than a truck size — modest width, modest height, a sidewall short enough for a car-derived platform and tall enough to survive a kerb. Draw it against the 70-series and the two outlines share a rim line and separate cleanly at the crown, which is what a pure aspect-ratio change looks like.