245/40R20 vs 255/45R20
255/45R20 is outside the ±3% rule as a replacement for 245/40R20: the diameter differs by +4.76%, 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 245/40R20, the speedometer reads 57.3 mph when you're really doing 60, and the odometer under-counts by 4.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.4 mph |
| 45 mph | 47.1 mph |
| 60 mph | 62.9 mph |
| 70 mph | 73.3 mph |
| 80 mph | 83.8 mph |
Other sizes close to 245/40R20
- 245/40R20 vs 245/45R19-0.13%
- 245/40R20 vs 245/45R20+3.48%
- 245/40R20 vs 275/40R20+3.41%
- 245/40R20 vs 275/40R19-0.20%
- 245/40R20 vs 245/35R20-3.48%
- 245/40R20 vs 255/35R20-2.49%
- 245/40R20 vs 275/35R20-0.50%
- 245/40R20 vs 255/40R20+1.14%
- 245/40R20 vs 285/35R20+0.50%
- 245/40R20 vs 315/35R20+3.48%
- 245/40R20 vs 285/30R20-3.55%
- 245/40R20 vs 275/30R20-4.40%
Questions
Can I replace 245/40R20 with 255/45R20?
255/45R20 is outside the ±3% rule as a replacement for 245/40R20: the diameter differs by +4.76%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 245/40R20 and 255/45R20?
255/45R20 is 1.32" taller and 10 mm wider; the sidewall is +0.66". At an indicated 60 mph you'd actually be doing 62.9 mph.
Will 255/45R20 fit on 245/40R20 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.