225/65R17 vs 245/65R17 — 20 mm that buys an inch of height

Two sizes, one aspect ratio, and a full inch of extra height that nobody asked for.

Why a wider tire got taller

The 65 in both codes is not a measurement. It is an instruction: make the sidewall 65% of the section width. Widen the casing from 225 mm to 245 mm and that instruction quietly issues a taller sidewall as well — 5.76 in becomes 6.27 in — and the sidewall counts twice, once above the wheel and once below.

So 20 mm of width returns 1.02 in of diameter. The two changes are not independent and cannot be separated while the aspect ratio is held: on this ladder, wider is taller, and the only way to add width without height is to drop the aspect ratio at the same time.

The percentage that stops being ignorable

3.59%. That is the interesting threshold on this pair, because it is roughly where speedometer error crosses out of instrument tolerance and becomes something a phone's GPS will show you — a repeatable gap at a steady cruise rather than noise.

Odometer drift follows, and over a service interval it amounts to a few hundred miles of unrecorded use. That matters more to a maintenance schedule than to a speeding ticket, which is the opposite of the way most people rank the two.

What the 0.51 in at the axle is for

Ride height rises by half an inch, because half the diameter change happens below the wheel centre. That is the entire mechanical reason this size is associated with lifted Outbacks: the tire is usually fitted alongside a spacer lift, and the tire itself contributes about half an inch of the total. People frequently attribute the whole change to the kit.

Underbody clearance and axle-centre height are different quantities, and only the second one is a property of the tire. What sits under the differential is unchanged by any of this.

The step below it

If the inch is more than you want, the cautious version is 225/65R17 to 235/65R17 — half the width increment, and a diameter change small enough that most people would not chase it. It is the same arithmetic with the 20 mm halved, and it is the reason two adjacent sizes on the same shelf can feel like very different propositions.

The thing a car-based platform cares about most

Not diameter. Width. 8.86 in of casing becomes 9.65 in, and on a unibody crossover the space between the tire and the strut, and between the tire and the wheel-arch liner at full lock, is usually measured in low single-digit inches to begin with. Nothing on this site knows what those gaps are on any particular car — 0.79 in of extra nominal section is the whole of the measurable claim, and even that figure moves with the wheel, since a casing fitted narrower than its reference rim draws its shoulders in and stands fractionally over the 29.54 in computed above.

The count that drives everything else

Each wheel turns 707.3 times per mile on the smaller size and 682.8 times on the larger — about 24 fewer revolutions. That single figure is where the speedometer error, the odometer drift and the change in engine speed at a fixed road speed all come from; they are three readings of one underlying quantity rather than three separate effects. The revolutions per mile reference works through why.

What you would still have to check

The load index. A 245/65R17 is stocked across a broad range of ratings because it is sold for crossovers and light trucks alike, and the two ends of that range are hundreds of pounds apart per tire. The figure your vehicle needs is printed on the door-jamb placard, and it does not change when the tire does.

Nothing on this page is a clearance finding. It is two size codes worked through to their consequences, and the consequences stop at the arithmetic. If your car started life a step lower still, on a 60-series, then 225/60R17 against this same 245/65R17 is the version of the sum that applies to you — and it is nearly twice as large.