305/30R19 vs 255/40R19
255/40R19 is outside the ±3% rule as a replacement for 305/30R19: the diameter differs by +3.16%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/40R19 on a car calibrated for 305/30R19, the speedometer reads 58.2 mph when you're really doing 60, and the odometer under-counts by 3.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.9 mph |
| 45 mph | 46.4 mph |
| 60 mph | 61.9 mph |
| 70 mph | 72.2 mph |
| 80 mph | 82.5 mph |
Other sizes close to 305/30R19
- 305/30R19 vs 225/45R18-0.89%
- 305/30R19 vs 235/45R18+0.47%
- 305/30R19 vs 245/40R19+1.95%
- 305/30R19 vs 235/40R19+0.75%
- 305/30R19 vs 255/35R19-0.68%
- 305/30R19 vs 255/40R18-0.66%
- 305/30R19 vs 275/35R19+1.43%
- 305/30R19 vs 245/35R19-1.73%
- 305/30R19 vs 225/40R19-0.45%
- 305/30R19 vs 235/35R19-2.78%
- 305/30R19 vs 225/45R19+2.93%
- 305/30R19 vs 285/35R19+2.48%
Questions
Can I replace 305/30R19 with 255/40R19?
255/40R19 is outside the ±3% rule as a replacement for 305/30R19: the diameter differs by +3.16%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 305/30R19 and 255/40R19?
255/40R19 is 0.83" taller and 50 mm narrower; the sidewall is +0.41". At an indicated 60 mph you'd actually be doing 61.9 mph.
Will 255/40R19 fit on 305/30R19 wheels?
Both use a 19" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.