295/30R18 vs 255/40R18
255/40R18 is outside the ±3% rule as a replacement for 295/30R18: the diameter differs by +4.26%, 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 295/30R18, the speedometer reads 57.5 mph when you're really doing 60, and the odometer under-counts by 4.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.3 mph |
| 45 mph | 46.9 mph |
| 60 mph | 62.6 mph |
| 70 mph | 73.0 mph |
| 80 mph | 83.4 mph |
Other sizes close to 295/30R18
- 295/30R18 vs 225/45R17+0.02%
- 295/30R18 vs 245/40R18+3.00%
- 295/30R18 vs 225/45R18+4.02%
- 295/30R18 vs 225/40R18+0.47%
- 295/30R18 vs 205/50R17+0.41%
- 295/30R18 vs 235/40R18+1.73%
- 295/30R18 vs 255/35R18+0.24%
- 295/30R18 vs 275/35R18+2.44%
- 295/30R18 vs 265/35R18+1.34%
- 295/30R18 vs 255/40R17+0.25%
- 295/30R18 vs 215/45R18+2.60%
- 295/30R18 vs 245/35R18-0.87%
Questions
Can I replace 295/30R18 with 255/40R18?
255/40R18 is outside the ±3% rule as a replacement for 295/30R18: the diameter differs by +4.26%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 295/30R18 and 255/40R18?
255/40R18 is 1.06" taller and 40 mm narrower; the sidewall is +0.53". At an indicated 60 mph you'd actually be doing 62.6 mph.
Will 255/40R18 fit on 295/30R18 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.