Subaru Forester 5th gen: 6.91% is a big step for a crossover

The starting point is small

225/60R17 measures 27.63 in and 225/55R18 measures 27.74 in — 0.11 in apart, on the same 8.86 in section width. These are among the smallest factory diameters recorded on this site, and that matters for everything below: on a small tire, a given number of inches is a larger percentage.

Sidewall runs 5.31 in on the 17-inch fitment and 4.87 in on the 18.

Two upgrades at completely different scales

• 235/60R17 at 28.10 in — +0.47 in, 1.71% from 225/60R17. A conventional one-width step.
• 245/65R17 at 29.54 in — +1.91 in, 6.91%, with +0.79 in of width and a full inch more sidewall at 6.27 in.

The gap between those two is the interesting part. The first is a substitution; the second is a real change, and 6.91% on a crossover is larger than what many light-truck owners do to their vehicles.

At 6.91%, an indicated 70 mph is 74.8 and 100 miles of odometer is 106.9 travelled. That is enough to matter on a long journey and enough that recalibrating the stored tire size, where the vehicle permits it, stops being pedantry.

What the taller tire is really buying

Nearly a full inch of extra sidewall — 5.31 in to 6.27 in — on a vehicle with modest ground clearance and a suspension designed for it. Ride height at the axle rises 0.95 in, which is half the diameter change and the only half that increases clearance under the vehicle.

The other consequence is unsprung mass. A 245/65R17 is a considerably heavier tire than a 225/60R17, and on a light vehicle with a small engine that is felt in acceleration and in fuel consumption more than it would be on a truck.

Circumference matching on permanent all-wheel drive

This platform drives all four wheels permanently, and the driveline expects the four rolling circumferences to stay close to each other. A size change applied to all four keeps them matched; mixing sizes, or running a spare of the original size after upgrading, does not.

The size of the mismatch a spare would introduce is exactly the percentage above. Worth knowing before it becomes relevant at the roadside rather than after.

From the 18-inch trim

245/65R17 measured from 225/55R18 is +1.80 in and 6.47%, and it involves changing wheels as well as tires. 235/60R17 from the same start is +0.36 in and 1.29%.

Owners moving off the 18-inch wheel are usually after the sidewall rather than the diameter, and the arithmetic supports that reading: the 17-inch factory size alone is 0.41% shorter than the 18-inch one while carrying 0.44 in more sidewall.

Where the two upgrades come from

The size gap between them is not arbitrary. 235/60R17 is the next step along the same series — one width increment, same aspect ratio — and it behaves exactly as that arithmetic predicts.

245/65R17 changes both numbers at once, moving up a width increment and five points of aspect ratio, and the two changes compound: the sidewall is 65% of a bigger number, so height grows twice over. That is why the second option is four times the change of the first despite being only one size code away from it.

It is a useful demonstration of a general rule. Changing the width alone moves diameter modestly; changing width and aspect ratio together moves it a great deal, and the aspect ratio page works through why.

The ratio column, and a symmetrical-drive complication

Nothing above needs a ratio; every figure is geometry. Where it would matter on this vehicle is subtler than on a truck: with drive permanently at all four wheels, a change in rolling circumference is not just a speedometer question, it is a question about what the centre differential is being asked to absorb — and that only stays benign while all four tires match.

Where this stops

None of it says a tire fits. Strut clearance, liner clearance at full lock and the placard's load requirement are three separate questions about your own vehicle, and a 6.91% change on a crossover touches all three at once — the last of them because a taller, wider casing on an unchanged rim is not automatically rated for what the placard asks of the tire it replaces.