265/70R17 vs 275/70R17 — is 10 mm worth doing?

10 mm is the smallest step the metric size system offers. It is worth knowing what it buys before paying for it.

The whole change, in two lines

Section width: 10.43 in to 10.83 in. That is 0.39 in — call it three-eighths of an inch spread across the whole casing, so a little under three-sixteenths per sidewall.

Overall diameter: 31.61 in to 32.16 in, a gain of 0.55 in.

Look at those two numbers in that order and something odd emerges. The tire got taller by more than it got wider. That is not a typing error and it is not unusual — it is what the aspect ratio does.

Why the height moved more than the width

70-series means the sidewall is 70% of the section width. Add 10 mm of width and you add 7 mm of sidewall with it, automatically. Then the sidewall gets counted twice — once at the top of the tire and once at the bottom — so 7 mm of sidewall becomes 14 mm of diameter, which is 0.55 in.

The width, meanwhile, is counted once. 10 mm is 0.39 in and stays 0.39 in.

So on any pairing where the aspect ratio is held constant, the diameter always moves more than the width does, by a factor of 2 × the aspect ratio ÷ 100 — 1.4 in this case. That relationship holds across the whole metric system and is worth carrying around.

Whether 1.74% matters

Not to the speedometer. The needle runs about 1.2 mph light at motorway pace and the odometer gives up 1.7 miles in every hundred. Both are inside the range that tread wear alone will produce over the life of a set: a tire worn from new to its wear bars loses roughly half an inch of diameter, which on these sizes is comparable to the entire difference between them.

That last comparison is the honest way to size up any change under two per cent. If the swap is smaller than the drift you already accept from wear, it is not a swap that needs accounting for.

Why a 4Runner owner asks anyway

Because 275/70R17 is very often the smallest step that is actually available in the tread pattern someone wants. The decision is rarely "should I add 10 mm"; it is "the tire I want is made in 275 and not in 265", and this page exists to confirm that taking it costs 0.55 in of height and nothing of consequence anywhere else.

Fitment guides list both sizes for the same wheels, and the extra 0.39 in of casing sits mostly inside the space the 265 already occupied.

The step that is not this one

If 10 mm is too small to bother with, the next rung is 285/70R17 — the same wheel, the same aspect ratio, 20 mm wider — at 1.10 in of diameter, exactly double this one, because the arithmetic above is linear. Two 10 mm steps and one 20 mm step produce the same answer.

Revolutions per mile, for the record

638.1 becomes 627.2, eleven rotations fewer in a mile. Every downstream figure on this page is that one ratio expressed differently, including the engine speed at a fixed road speed, which drops by the same 1.7%.

The distance per turn

Circumference goes from 99.3 in to 101.0 in — the wheel covers about an inch and three-quarters more ground with each rotation. That is the physical quantity a speedometer is actually calibrated against; the diameter is only how the size code expresses it. Everything above follows from those two circumference figures and from nothing else.

What is still unknown

Whether 0.39 in of extra section width clears whatever is closest to your tire at full lock, and whether the wheel's offset places that extra width inboard, outboard, or split between the two. Both are established by putting the vehicle on the ground and turning the steering to its stops. The geometry above is complete and it is only geometry.