235/45R20 vs 305/40R20
305/40R20 is outside the ±3% rule as a replacement for 235/45R20: the diameter differs by +4.52%, 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 235/45R20, the speedometer reads 57.4 mph when you're really doing 60, and the odometer under-counts by 4.5%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.4 mph |
| 45 mph | 47.0 mph |
| 60 mph | 62.7 mph |
| 70 mph | 73.2 mph |
| 80 mph | 83.6 mph |
Other sizes close to 235/45R20
- 235/45R20 vs 245/45R20+1.25%
- 235/45R20 vs 255/45R20+2.50%
- 235/45R20 vs 275/40R20+1.18%
- 235/45R20 vs 245/40R20-2.15%
- 235/45R20 vs 245/50R20+4.66%
- 235/45R20 vs 255/35R20-4.59%
- 235/45R20 vs 255/40R20-1.04%
- 235/45R20 vs 275/35R20-2.64%
- 235/45R20 vs 235/50R19-0.26%
- 235/45R20 vs 265/45R20+3.75%
- 235/45R20 vs 285/35R20-1.67%
- 235/45R20 vs 315/35R20+1.25%
Questions
Can I replace 235/45R20 with 305/40R20?
305/40R20 is outside the ±3% rule as a replacement for 235/45R20: the diameter differs by +4.52%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 235/45R20 and 305/40R20?
305/40R20 is 1.28" taller and 70 mm wider; the sidewall is +0.64". At an indicated 60 mph you'd actually be doing 62.7 mph.
Will 305/40R20 fit on 235/45R20 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.