265/70R17 vs 245/75R17 — narrower without getting shorter

Almost every comparison on this site is about going bigger. This one is about going narrower on purpose and getting away with it.

The substitution

Take 20 mm out of the section width and put five points back into the aspect ratio. The casing shrinks from 10.43 in to 9.65 in — 0.79 in, or 7.55% narrower — and the sidewall barely notices, going from 7.30 in to 7.23 in, because 75% of a smaller number lands almost exactly where 70% of a larger one did.

Net effect on rolling diameter: 0.14 in shorter. As a percentage, 0.44%.

That is a smaller change than the manufacturing variation between two brands of the same size. In practical terms the two tires roll identically, and any system in the vehicle that counts wheel revolutions carries on without noticing that anything happened.

Why anybody wants a narrower tire

Winter fitments, mostly. A second set of wheels bought for cold-weather use is very often specified narrower than the summer set, and the buyer's constraint is that the vehicle must not need recalibrating twice a year. This pair satisfies that constraint about as well as it can be satisfied.

There is a second reason, less discussed: a 245-section tire in a given model line is frequently cheaper than the 265, and on a 17-inch wheel the narrower size is often available in tread patterns the wider one is not. Availability shapes more of these decisions than geometry does.

What the arithmetic does not claim

Nothing on this page says a narrower tire performs better in snow, or worse, or at all. Contact pressure, tread compound, siping and the vehicle's weight distribution decide that, and none of them is derivable from a size code. What this page establishes is only that the substitution is geometrically free — the diameter is held, so whatever difference you experience is a difference in the tire, not in the gearing.

Reading the direction of the error

The narrower tire is the shorter one here, despite carrying five more aspect points, so a speedometer calibrated for 265/70R17 reads very slightly high on 245/75R17. Over a hundred recorded miles the odometer over-counts by less than half a mile — which is a smaller quantity than the difference between two brands of the same size.

Both figures are small enough to be noise. They are stated here because the sign matters conceptually even when the size does not — this is one of the few pairings in the set where the second size is the smaller one, and it is easy to read a table of positive numbers out of habit.

The sidewall barely moves, and that is the point

7.30 in against 7.23 in — seven hundredths of an inch. Two tires with visibly different footprints and effectively identical sidewall height. If you have been told that a taller aspect ratio means a taller tire, this pairing is the counter-example: 75 is a bigger percentage of a smaller number, and the two changes cancel almost exactly.

That cancellation is not luck. Tire manufacturers build size ladders so that adjacent widths can be substituted at constant diameter, because fleet buyers and winter-package buyers need exactly that.

Where to look next

The same starting size going the other way — 265/70R17 against 275/70R17 — adds 10 mm of width while holding the aspect ratio, and gains 0.55 in of height for it. Set that against the 0.14 in lost here and the mechanism is obvious: holding the aspect ratio makes width and height move together, and trading the aspect ratio against the width is what lets you separate them.

What still has to be checked on the vehicle

Rim width — the dimension neither code carries. Dropping 20 mm of nominal section while the wheels stay put means the new casing is being stretched over a wheel chosen with the old one in mind, and a casing stretched wider than its own reference measures wider and crowns lower, which trims a little off the 31.47 in computed above. How much depends on what the wheels actually are and what the tire's data sheet approves, neither of which is recorded here; the rim width reference gives the rate of change, about 0.2 in of section per half inch of wheel.