235/40R18 vs 235/45R18
235/45R18 sits +3.6% away from 235/40R18 on rolling diameter — past the 3% rule but short of 5%. It will physically fit many cars, but you are trading measurable speedometer and odometer error for the look. Budget 11.8 mm of extra clearance per side, and check the inner arch at full lock before committing. On a car with torque-vectoring or adaptive cruise, have the wheel-speed calibration checked.
- Diameter change
- +3.64%
- +23.5 mm
- Ride height
- +11.8 mm
- half the diameter change
- Tread width
- 0 mm
- 235 mm section
- Speedo at 60
- 62.2
- true mph when the dial reads 60
Side by side
| Measure | 235/40R18 | 235/45R18 | Change |
|---|---|---|---|
| Overall diameter | 25.40″ | 26.33″ | +3.64% |
| Section width | 235 mm | 235 mm | 0 mm |
| Sidewall height | 94.0 mm | 105.8 mm | +11.8 mm |
| Rim diameter | 18″ | 18″ | 0″ |
| Revolutions / mile | 794 | 766 | −28 |
Questions about this swap
›Can I fit 235/45R18 instead of 235/40R18?
Only with care. The diameter differs by +3.6%, outside the 3% rule, so clearance and speedometer accuracy both need checking first.
›How much will my speedometer be out with 235/45R18 instead of 235/40R18?
By about 3.6%. At an indicated 60 mph you would actually be doing 62.2 mph, so the dial under-reads your true speed.
›Does fitting 235/45R18 change ride height from 235/40R18?
Yes, by roughly 11.8 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 235/45R18 rather than 235/40R18?
It will read 3.6% low. Over 10,000 recorded miles that is about 364 miles of difference against the true distance travelled.
Want to try other sizes? Open the calculator.