305/25R20 vs 305/30R20
305/30R20 is outside the ±3% rule as a replacement for 305/25R20: the diameter differs by +4.62%, 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 305/25R20, the speedometer reads 57.4 mph when you're really doing 60, and the odometer under-counts by 4.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.4 mph |
| 45 mph | 47.1 mph |
| 60 mph | 62.8 mph |
| 70 mph | 73.2 mph |
| 80 mph | 83.7 mph |
Other sizes close to 305/25R20
- 305/25R20 vs 255/35R19+0.09%
- 305/25R20 vs 245/35R20+2.88%
- 305/25R20 vs 255/35R20+3.94%
- 305/25R20 vs 245/35R19-0.97%
- 305/25R20 vs 225/40R19+0.32%
- 305/25R20 vs 285/30R20+2.80%
- 305/25R20 vs 275/30R20+1.89%
- 305/25R20 vs 265/35R20+5.00%
- 305/25R20 vs 235/35R20+1.82%
- 305/25R20 vs 295/30R20+3.71%
- 305/25R20 vs 305/30R19+0.77%
- 305/25R20 vs 295/30R19-0.14%
Questions
Can I replace 305/25R20 with 305/30R20?
305/30R20 is outside the ±3% rule as a replacement for 305/25R20: the diameter differs by +4.62%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 305/25R20 and 305/30R20?
305/30R20 is 1.20" taller and 0 mm wider; the sidewall is +0.60". At an indicated 60 mph you'd actually be doing 62.8 mph.
Will 305/30R20 fit on 305/25R20 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.