245/40R18 vs 255/45R18
255/45R18 is outside the ±3% rule as a replacement for 245/40R18: the diameter differs by +5.13%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/45R18 on a car calibrated for 245/40R18, the speedometer reads 57.1 mph when you're really doing 60, and the odometer under-counts by 5.1%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.5 mph |
| 45 mph | 47.3 mph |
| 60 mph | 63.1 mph |
| 70 mph | 73.6 mph |
| 80 mph | 84.1 mph |
Other sizes close to 245/40R18
- 245/40R18 vs 225/45R18+1.00%
- 245/40R18 vs 225/40R18-2.45%
- 245/40R18 vs 225/50R17+0.55%
- 245/40R18 vs 245/45R18+3.75%
- 245/40R18 vs 235/45R18+2.37%
- 245/40R18 vs 235/40R18-1.22%
- 245/40R18 vs 225/50R18+4.44%
- 245/40R18 vs 215/50R17-0.98%
- 245/40R18 vs 245/45R17-0.14%
- 245/40R18 vs 255/35R18-2.68%
- 245/40R18 vs 255/40R18+1.22%
- 245/40R18 vs 245/35R19+0.14%
Questions
Can I replace 245/40R18 with 255/45R18?
255/45R18 is outside the ±3% rule as a replacement for 245/40R18: the diameter differs by +5.13%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 245/40R18 and 255/45R18?
255/45R18 is 1.32" taller and 10 mm wider; the sidewall is +0.66". At an indicated 60 mph you'd actually be doing 63.1 mph.
Will 255/45R18 fit on 245/40R18 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.