255/30R20 vs 305/30R20
305/30R20 is outside the ±3% rule as a replacement for 255/30R20: the diameter differs by +4.54%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 305/30R20 on a car calibrated for 255/30R20, the speedometer reads 57.4 mph when you're really doing 60, and the odometer under-counts by 4.5%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.4 mph |
| 45 mph | 47.0 mph |
| 60 mph | 62.7 mph |
| 70 mph | 73.2 mph |
| 80 mph | 83.6 mph |
Other sizes close to 255/30R20
- 255/30R20 vs 255/35R19+0.02%
- 255/30R20 vs 245/35R20+2.80%
- 255/30R20 vs 255/35R20+3.86%
- 255/30R20 vs 225/40R19+0.24%
- 255/30R20 vs 285/30R20+2.72%
- 255/30R20 vs 275/30R20+1.82%
- 255/30R20 vs 265/35R20+4.92%
- 255/30R20 vs 235/35R20+1.74%
- 255/30R20 vs 295/30R20+3.63%
- 255/30R20 vs 305/30R19+0.70%
- 255/30R20 vs 295/30R19-0.21%
- 255/30R20 vs 245/30R20-0.91%
Questions
Can I replace 255/30R20 with 305/30R20?
305/30R20 is outside the ±3% rule as a replacement for 255/30R20: the diameter differs by +4.54%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 255/30R20 and 305/30R20?
305/30R20 is 1.18" taller and 50 mm wider; the sidewall is +0.59". At an indicated 60 mph you'd actually be doing 62.7 mph.
Will 305/30R20 fit on 255/30R20 wheels?
Both use a 20" rim, so the wheel diameter matches; check the wheel's width range for a 305 mm tire and the fender clearance.