255/40R19 vs 235/45R18
Fitting 235/45R18 in place of 255/40R19 changes rolling diameter by −2.6%, which is inside the 3% tolerance the industry treats as a safe substitution. The tyre is shorter and 20 mm narrower, so you need roughly 8.9 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
- −2.61%
- −17.9 mm
- Ride height
- −8.9 mm
- half the diameter change
- Tread width
- −20 mm
- 235 mm section
- Speedo at 60
- 58.4
- true mph when the dial reads 60
Side by side
| Measure | 255/40R19 | 235/45R18 | Change |
|---|---|---|---|
| Overall diameter | 27.03″ | 26.33″ | −2.61% |
| Section width | 255 mm | 235 mm | −20 mm |
| Sidewall height | 102.0 mm | 105.8 mm | +3.8 mm |
| Rim diameter | 19″ | 18″ | −1″ |
| Revolutions / mile | 746 | 766 | +20 |
Questions about this swap
›Can I fit 235/45R18 instead of 255/40R19?
Yes, in most cases. The −2.6% 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 235/45R18 instead of 255/40R19?
By about 2.6%. At an indicated 60 mph you would actually be doing 58.4 mph, so the dial over-reads your true speed.
›Does fitting 235/45R18 change ride height from 255/40R19?
Yes, by roughly 8.9 mm. Rolling radius is half the diameter change, so the axle sits that much lower and you need the same amount of extra room above the tyre.
›Will my odometer read differently on 235/45R18 rather than 255/40R19?
It will read 2.6% high. Over 10,000 recorded miles that is about 261 miles of difference against the true distance travelled.
Want to try other sizes? Open the calculator.