255/40R18 vs 305/30R19
305/30R19 is a safe swap for 255/40R18 by the usual ±3% rule: the diameter differs by +0.67%.
Speedometer and odometer
With 305/30R19 on a car calibrated for 255/40R18, the speedometer reads 59.6 mph when you're really doing 60, and the odometer under-counts by 0.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.4 mph |
| 70 mph | 70.5 mph |
| 80 mph | 80.5 mph |
Other sizes close to 255/40R18
- 255/40R18 vs 225/45R18-0.23%
- 255/40R18 vs 245/40R18-1.21%
- 255/40R18 vs 225/40R18-3.63%
- 255/40R18 vs 225/50R17-0.67%
- 255/40R18 vs 245/45R18+2.50%
- 255/40R18 vs 235/50R18+4.69%
- 255/40R18 vs 235/45R18+1.13%
- 255/40R18 vs 235/40R18-2.42%
- 255/40R18 vs 225/50R18+3.18%
- 255/40R18 vs 255/35R19-0.02%
- 255/40R18 vs 255/35R18-3.86%
- 255/40R18 vs 225/40R19+0.21%
Questions
Can I replace 255/40R18 with 305/30R19?
305/30R19 is a safe swap for 255/40R18 by the usual ±3% rule: the diameter differs by +0.67%.
What's the difference between 255/40R18 and 305/30R19?
305/30R19 is 0.17" taller and 50 mm wider; the sidewall is -0.41". At an indicated 60 mph you'd actually be doing 60.4 mph.
Will 305/30R19 fit on 255/40R18 wheels?
No — 255/40R18 needs a 18" wheel and 305/30R19 a 19" wheel. This is a plus/minus-size swap that changes wheels too.