255/45R20 vs 305/40R20
305/40R20 is a safe swap for 255/45R20 by the usual ±3% rule: the diameter differs by +1.97%.
Speedometer and odometer
With 305/40R20 on a car calibrated for 255/45R20, the speedometer reads 58.8 mph when you're really doing 60, and the odometer under-counts by 2.0%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.6 mph |
| 45 mph | 45.9 mph |
| 60 mph | 61.2 mph |
| 70 mph | 71.4 mph |
| 80 mph | 81.6 mph |
Other sizes close to 255/45R20
- 255/45R20 vs 235/55R19+0.49%
- 255/45R20 vs 245/45R20-1.22%
- 255/45R20 vs 255/50R19+0.01%
- 255/45R20 vs 275/40R20-1.29%
- 255/45R20 vs 255/50R20+3.46%
- 255/45R20 vs 245/40R20-4.54%
- 255/45R20 vs 265/50R20+4.81%
- 255/45R20 vs 245/50R20+2.10%
- 255/45R20 vs 275/45R20+2.44%
- 255/45R20 vs 235/55R20+3.93%
- 255/45R20 vs 255/40R20-3.46%
- 255/45R20 vs 265/45R20+1.22%
Questions
Can I replace 255/45R20 with 305/40R20?
305/40R20 is a safe swap for 255/45R20 by the usual ±3% rule: the diameter differs by +1.97%.
What's the difference between 255/45R20 and 305/40R20?
305/40R20 is 0.57" taller and 50 mm wider; the sidewall is +0.29". At an indicated 60 mph you'd actually be doing 61.2 mph.
Will 305/40R20 fit on 255/45R20 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.