225/45R18 vs 255/40R18
255/40R18 is a safe swap for 225/45R18 by the usual ±3% rule: the diameter differs by +0.23%.
Speedometer and odometer
With 255/40R18 on a car calibrated for 225/45R18, the speedometer reads 59.9 mph when you're really doing 60, and the odometer under-counts by 0.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.1 mph |
| 60 mph | 60.1 mph |
| 70 mph | 70.2 mph |
| 80 mph | 80.2 mph |
Other sizes close to 225/45R18
- 225/45R18 vs 245/40R18-0.99%
- 225/45R18 vs 225/40R18-3.41%
- 225/45R18 vs 225/50R17-0.44%
- 225/45R18 vs 245/45R18+2.73%
- 225/45R18 vs 235/50R18+4.93%
- 225/45R18 vs 235/45R18+1.36%
- 225/45R18 vs 235/40R18-2.20%
- 225/45R18 vs 225/50R18+3.41%
- 225/45R18 vs 255/35R19+0.21%
- 225/45R18 vs 255/35R18-3.64%
- 225/45R18 vs 245/35R19-0.85%
- 225/45R18 vs 225/40R19+0.44%
Questions
Can I replace 225/45R18 with 255/40R18?
255/40R18 is a safe swap for 225/45R18 by the usual ±3% rule: the diameter differs by +0.23%.
What's the difference between 225/45R18 and 255/40R18?
255/40R18 is 0.06" taller and 30 mm wider; the sidewall is +0.03". At an indicated 60 mph you'd actually be doing 60.1 mph.
Will 255/40R18 fit on 225/45R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.