255/40R19 vs 305/30R20
305/30R20 is a safe swap for 255/40R19 by the usual ±3% rule: the diameter differs by +0.64%.
Speedometer and odometer
With 305/30R20 on a car calibrated for 255/40R19, the speedometer reads 59.6 mph when you're really doing 60, and the odometer under-counts by 0.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.4 mph |
| 70 mph | 70.4 mph |
| 80 mph | 80.5 mph |
Other sizes close to 255/40R19
- 255/40R19 vs 235/50R18+0.82%
- 255/40R19 vs 245/45R19+2.40%
- 255/40R19 vs 245/40R19-1.17%
- 255/40R19 vs 225/50R18-0.64%
- 255/40R19 vs 235/40R19-2.33%
- 255/40R19 vs 255/35R19-3.71%
- 255/40R19 vs 255/45R19+3.71%
- 255/40R19 vs 275/35R19-1.67%
- 255/40R19 vs 275/40R19+2.33%
- 255/40R19 vs 245/35R19-4.73%
- 255/40R19 vs 255/35R20-0.01%
- 255/40R19 vs 225/40R19-3.50%
Questions
Can I replace 255/40R19 with 305/30R20?
305/30R20 is a safe swap for 255/40R19 by the usual ±3% rule: the diameter differs by +0.64%.
What's the difference between 255/40R19 and 305/30R20?
305/30R20 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 305/30R20 fit on 255/40R19 wheels?
No — 255/40R19 needs a 19" wheel and 305/30R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.