255/40R20 vs 255/45R20
255/45R20 is outside the ±3% rule as a replacement for 255/40R20: the diameter differs by +3.58%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/45R20 on a car calibrated for 255/40R20, the speedometer reads 57.9 mph when you're really doing 60, and the odometer under-counts by 3.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.1 mph |
| 45 mph | 46.6 mph |
| 60 mph | 62.1 mph |
| 70 mph | 72.5 mph |
| 80 mph | 82.9 mph |
Other sizes close to 255/40R20
- 255/40R20 vs 245/45R20+2.32%
- 255/40R20 vs 275/40R20+2.25%
- 255/40R20 vs 245/40R20-1.12%
- 255/40R20 vs 255/45R19+0.01%
- 255/40R20 vs 245/35R20-4.56%
- 255/40R20 vs 255/35R20-3.58%
- 255/40R20 vs 275/35R20-1.62%
- 255/40R20 vs 235/50R19+0.79%
- 255/40R20 vs 265/45R20+4.85%
- 255/40R20 vs 285/35R20-0.63%
- 255/40R20 vs 315/35R20+2.32%
- 255/40R20 vs 285/30R20-4.63%
Questions
Can I replace 255/40R20 with 255/45R20?
255/45R20 is outside the ±3% rule as a replacement for 255/40R20: the diameter differs by +3.58%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 255/40R20 and 255/45R20?
255/45R20 is 1.00" taller and 0 mm wider; the sidewall is +0.50". At an indicated 60 mph you'd actually be doing 62.1 mph.
Will 255/45R20 fit on 255/40R20 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.