225/45R17 vs 235/40R18
Fitting 235/40R18 in place of 225/45R17 changes rolling diameter by +1.7%, which is inside the 3% tolerance the industry treats as a safe substitution. The tyre is taller and 10 mm wider, so you need roughly 5.5 mm of extra room at the top of the arch and at the strut. Speedometer error stays small enough that most drivers never notice it, and ABS and traction control will continue to work from calibration.
- Diameter change
- +1.72%
- +10.9 mm
- Ride height
- +5.5 mm
- half the diameter change
- Tread width
- +10 mm
- 235 mm section
- Speedo at 60
- 61.0
- true mph when the dial reads 60
Side by side
| Measure | 225/45R17 | 235/40R18 | Change |
|---|---|---|---|
| Overall diameter | 24.97″ | 25.40″ | +1.72% |
| Section width | 225 mm | 235 mm | +10 mm |
| Sidewall height | 101.3 mm | 94.0 mm | −7.3 mm |
| Rim diameter | 17″ | 18″ | +1″ |
| Revolutions / mile | 808 | 794 | −14 |
Questions about this swap
›Can I fit 235/40R18 instead of 225/45R17?
Yes, in most cases. The +1.7% diameter change is inside the 3% tolerance for a safe substitution, provided load index and speed rating match or exceed the original.
›How much will my speedometer be out with 235/40R18 instead of 225/45R17?
By about 1.7%. At an indicated 60 mph you would actually be doing 61.0 mph, so the dial under-reads your true speed.
›Does fitting 235/40R18 change ride height from 225/45R17?
Yes, by roughly 5.5 mm. Rolling radius is half the diameter change, so the axle sits that much higher and you need the same amount of extra room above the tyre.
›Will my odometer read differently on 235/40R18 rather than 225/45R17?
It will read 1.7% low. Over 10,000 recorded miles that is about 172 miles of difference against the true distance travelled.
Want to try other sizes? Open the calculator.