255/40R18 vs 235/45R18
235/45R18 is a safe swap for 255/40R18 by the usual ±3% rule: the diameter differs by +1.13%.
Speedometer and odometer
With 235/45R18 on a car calibrated for 255/40R18, the speedometer reads 59.3 mph when you're really doing 60, and the odometer under-counts by 1.1%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.3 mph |
| 45 mph | 45.5 mph |
| 60 mph | 60.7 mph |
| 70 mph | 70.8 mph |
| 80 mph | 80.9 mph |
Other sizes close to 255/40R18
- 255/40R18 vs 225/45R18-0.23%
- 255/40R18 vs 245/40R18-1.21%
- 255/40R18 vs 225/40R18-3.63%
- 255/40R18 vs 225/50R17-0.67%
- 255/40R18 vs 245/45R18+2.50%
- 255/40R18 vs 235/50R18+4.69%
- 255/40R18 vs 235/40R18-2.42%
- 255/40R18 vs 225/50R18+3.18%
- 255/40R18 vs 255/35R19-0.02%
- 255/40R18 vs 255/35R18-3.86%
- 255/40R18 vs 225/40R19+0.21%
- 255/40R18 vs 275/35R18-1.74%
Questions
Can I replace 255/40R18 with 235/45R18?
235/45R18 is a safe swap for 255/40R18 by the usual ±3% rule: the diameter differs by +1.13%.
What's the difference between 255/40R18 and 235/45R18?
235/45R18 is 0.30" taller and 20 mm narrower; the sidewall is +0.15". At an indicated 60 mph you'd actually be doing 60.7 mph.
Will 235/45R18 fit on 255/40R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 235 mm tire and the fender clearance.