255/35R18 vs 245/40R18
245/40R18 is a safe swap for 255/35R18 by the usual ±3% rule: the diameter differs by +2.75%.
Speedometer and odometer
With 245/40R18 on a car calibrated for 255/35R18, the speedometer reads 58.4 mph when you're really doing 60, and the odometer under-counts by 2.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.8 mph |
| 45 mph | 46.2 mph |
| 60 mph | 61.7 mph |
| 70 mph | 71.9 mph |
| 80 mph | 82.2 mph |
Other sizes close to 255/35R18
- 255/35R18 vs 225/45R17-0.22%
- 255/35R18 vs 225/45R18+3.78%
- 255/35R18 vs 225/40R18+0.24%
- 255/35R18 vs 205/50R17+0.17%
- 255/35R18 vs 235/40R18+1.49%
- 255/35R18 vs 255/40R18+4.01%
- 255/35R18 vs 275/35R18+2.20%
- 255/35R18 vs 265/35R18+1.10%
- 255/35R18 vs 255/40R17+0.02%
- 255/35R18 vs 215/45R18+2.36%
- 255/35R18 vs 245/35R18-1.10%
- 255/35R18 vs 225/35R19+0.69%
Questions
Can I replace 255/35R18 with 245/40R18?
245/40R18 is a safe swap for 255/35R18 by the usual ±3% rule: the diameter differs by +2.75%.
What's the difference between 255/35R18 and 245/40R18?
245/40R18 is 0.69" taller and 10 mm narrower; the sidewall is +0.34". At an indicated 60 mph you'd actually be doing 61.7 mph.
Will 245/40R18 fit on 255/35R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 245 mm tire and the fender clearance.