305/40R20 vs 255/55R20
255/55R20 is outside the ±3% rule as a replacement for 305/40R20: the diameter differs by +4.85%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/55R20 on a car calibrated for 305/40R20, the speedometer reads 57.2 mph when you're really doing 60, and the odometer under-counts by 4.9%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.5 mph |
| 45 mph | 47.2 mph |
| 60 mph | 62.9 mph |
| 70 mph | 73.4 mph |
| 80 mph | 83.9 mph |
Other sizes close to 305/40R20
- 305/40R20 vs 245/45R20-3.12%
- 305/40R20 vs 255/45R20-1.93%
- 305/40R20 vs 255/50R20+1.46%
- 305/40R20 vs 275/40R20-3.19%
- 305/40R20 vs 265/50R20+2.79%
- 305/40R20 vs 245/50R20+0.13%
- 305/40R20 vs 275/45R20+0.47%
- 305/40R20 vs 235/55R20+1.93%
- 305/40R20 vs 245/55R19+0.01%
- 305/40R20 vs 265/45R20-0.73%
- 305/40R20 vs 265/50R19-0.59%
- 305/40R20 vs 315/35R20-3.12%
Questions
Can I replace 305/40R20 with 255/55R20?
255/55R20 is outside the ±3% rule as a replacement for 305/40R20: the diameter differs by +4.85%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 305/40R20 and 255/55R20?
255/55R20 is 1.44" taller and 50 mm narrower; the sidewall is +0.72". At an indicated 60 mph you'd actually be doing 62.9 mph.
Will 255/55R20 fit on 305/40R20 wheels?
Both use a 20" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.