245/40R18 vs 255/40R18
255/40R18 is a safe swap for 245/40R18 by the usual ±3% rule: the diameter differs by +1.22%.
Speedometer and odometer
With 255/40R18 on a car calibrated for 245/40R18, the speedometer reads 59.3 mph when you're really doing 60, and the odometer under-counts by 1.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.4 mph |
| 45 mph | 45.6 mph |
| 60 mph | 60.7 mph |
| 70 mph | 70.9 mph |
| 80 mph | 81.0 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 245/35R19+0.14%
- 245/40R18 vs 235/35R19-0.93%
Questions
Can I replace 245/40R18 with 255/40R18?
255/40R18 is a safe swap for 245/40R18 by the usual ±3% rule: the diameter differs by +1.22%.
What's the difference between 245/40R18 and 255/40R18?
255/40R18 is 0.31" taller and 10 mm wider; the sidewall is +0.16". At an indicated 60 mph you'd actually be doing 60.7 mph.
Will 255/40R18 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.