225/60R17 vs 235/60R17 — the smallest step the system offers
A Forester's most cautious upgrade, and a useful calibration point for how small a metric size step really is.
235/60R17 is 0.47 in taller than 225/60R17 (1.71%) and 0.39 in wider. A speedometer calibrated for 225/60R17 would under-read by 1.71%.
Measured against each other
Overall diameter
27.63 in28.10 in+0.47 in (+1.71%)Section width
8.86 in / 225 mm9.25 in / 235 mm+0.39 in (+4.44%)Sidewall height
5.31 in5.55 in+0.24 inCircumference
86.8 in88.3 in+1.5 inRevolutions per mile
729.9717.7-12.3Ride height at the axle
——+0.24 inSpeedometer reading
——+1.71% under-read
| Dimension | 225/60R17 | 235/60R17 | Change |
|---|---|---|---|
| Overall diameter | 27.63 in | 28.10 in | +0.47 in (+1.71%) |
| Section width | 8.86 in / 225 mm | 9.25 in / 235 mm | +0.39 in (+4.44%) |
| Sidewall height | 5.31 in | 5.55 in | +0.24 in |
| Circumference | 86.8 in | 88.3 in | +1.5 in |
| Revolutions per mile | 729.9 | 717.7 | -12.3 |
| Ride height at the axle | — | — | +0.24 in |
| Speedometer reading | — | — | +1.71% under-read |
Calibrated for 225/60R17, running 235/60R17
- Dial reads 30
- 30.5 mph
- Dial reads 50
- 50.9 mph
- Dial reads 70
- 71.2 mph
- Odometer counts 100
- 101.7 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
10 mm is the smallest increment the metric width scale contains. On a Forester it is also, in practice, the only step available before things start getting complicated.
What one increment buys
Section width goes from 8.86 in to 9.25 in. Sidewall goes from 5.31 in to 5.55 in, because 60% of a larger casing is a larger sidewall, and the diameter therefore rises 0.47 in to 28.10 in.
Everything downstream is smaller still. At a motorway 80 on the dial the car is doing 81.4, and that is the largest consequence the change has anywhere.
Why the change is so small and still worth a page
Because the alternative is much bigger. The other size Foresters are commonly fitted with is 245/65R17, which is a two-variable change and lands 1.91 in taller rather than half an inch. There is nothing between them. So the practical choice on this platform is between a step so small it barely registers and one almost four times its size, and knowing that shape of the decision is more useful than either number alone.
The reason people take it anyway
Tread availability. A 235/60R17 is offered in all-terrain and rugged patterns that 225/60R17 is not, and for an owner who wants a different kind of tire on the factory wheels this is often the smallest size that has one. The 0.47 in comes along with it rather than being the objective.
What a unibody crossover cares about
Width against the strut and the arch liner, mostly, and 0.39 in is a modest amount of it. What happens at the axle is worth stating in words rather than decimals: a quarter of an inch, which nobody would call a lift.
The multiplier, if you want to reuse it
At a fixed aspect ratio, the diameter always grows by twice the aspect ratio as a fraction of the width increment — here 1.2 times, since the aspect ratio is 60. So 0.39 in of extra width produces 0.47 in of extra height. On a 70-series pairing the same 10 mm would produce 0.55 in instead. The taller the profile, the more a width step costs in height.
The load index caveat
Foresters are light and their placard load index is correspondingly modest, but 235/60R17 is a size that spans car and light-crossover fitments and appears with a range of ratings. The requirement is printed inside the driver's door and is unaffected by either dimension above.