285/30R18 vs 255/40R18
255/40R18 is outside the ±3% rule as a replacement for 285/30R18: the diameter differs by +5.25%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/40R18 on a car calibrated for 285/30R18, the speedometer reads 57.0 mph when you're really doing 60, and the odometer under-counts by 5.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.6 mph |
| 45 mph | 47.4 mph |
| 60 mph | 63.2 mph |
| 70 mph | 73.7 mph |
| 80 mph | 84.2 mph |
Other sizes close to 285/30R18
- 285/30R18 vs 225/45R17+0.97%
- 285/30R18 vs 245/40R18+3.98%
- 285/30R18 vs 225/40R18+1.43%
- 285/30R18 vs 215/45R17-0.46%
- 285/30R18 vs 235/40R18+2.71%
- 285/30R18 vs 255/35R18+1.19%
- 285/30R18 vs 245/40R17-0.06%
- 285/30R18 vs 275/35R18+3.42%
- 285/30R18 vs 265/35R18+2.31%
- 285/30R18 vs 215/45R18+3.58%
- 285/30R18 vs 245/35R18+0.08%
- 285/30R18 vs 215/40R18+0.16%
Questions
Can I replace 285/30R18 with 255/40R18?
255/40R18 is outside the ±3% rule as a replacement for 285/30R18: the diameter differs by +5.25%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 285/30R18 and 255/40R18?
255/40R18 is 1.30" taller and 30 mm narrower; the sidewall is +0.65". At an indicated 60 mph you'd actually be doing 63.2 mph.
Will 255/40R18 fit on 285/30R18 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.