245/45R20 vs 305/40R20
305/40R20 is outside the ±3% rule as a replacement for 245/45R20: the diameter differs by +3.23%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 305/40R20 on a car calibrated for 245/45R20, the speedometer reads 58.1 mph when you're really doing 60, and the odometer under-counts by 3.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.0 mph |
| 45 mph | 46.5 mph |
| 60 mph | 61.9 mph |
| 70 mph | 72.3 mph |
| 80 mph | 82.6 mph |
Other sizes close to 245/45R20
- 245/45R20 vs 255/45R20+1.24%
- 245/45R20 vs 275/40R20-0.07%
- 245/45R20 vs 255/50R20+4.74%
- 245/45R20 vs 245/40R20-3.36%
- 245/45R20 vs 245/50R20+3.36%
- 245/45R20 vs 275/45R20+3.71%
- 245/45R20 vs 225/55R19+0.22%
- 245/45R20 vs 255/40R20-2.26%
- 245/45R20 vs 275/35R20-3.84%
- 245/45R20 vs 265/45R20+2.47%
- 245/45R20 vs 285/35R20-2.88%
- 245/45R20 vs 315/35R200.00%
Questions
Can I replace 245/45R20 with 305/40R20?
305/40R20 is outside the ±3% rule as a replacement for 245/45R20: the diameter differs by +3.23%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 245/45R20 and 305/40R20?
305/40R20 is 0.93" taller and 60 mm wider; the sidewall is +0.46". At an indicated 60 mph you'd actually be doing 61.9 mph.
Will 305/40R20 fit on 245/45R20 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.