305/35R20 vs 305/40R20
305/40R20 is outside the ±3% rule as a replacement for 305/35R20: the diameter differs by +4.23%, 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 305/35R20, the speedometer reads 57.6 mph when you're really doing 60, and the odometer under-counts by 4.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.3 mph |
| 45 mph | 46.9 mph |
| 60 mph | 62.5 mph |
| 70 mph | 73.0 mph |
| 80 mph | 83.4 mph |
Other sizes close to 305/35R20
- 305/35R20 vs 245/45R20+0.97%
- 305/35R20 vs 255/45R20+2.22%
- 305/35R20 vs 275/40R20+0.90%
- 305/35R20 vs 245/40R20-2.43%
- 305/35R20 vs 245/50R20+4.37%
- 305/35R20 vs 275/45R20+4.71%
- 305/35R20 vs 255/35R20-4.85%
- 305/35R20 vs 255/40R20-1.32%
- 305/35R20 vs 275/35R20-2.91%
- 305/35R20 vs 235/50R19-0.54%
- 305/35R20 vs 265/45R20+3.47%
- 305/35R20 vs 285/35R20-1.94%
Questions
Can I replace 305/35R20 with 305/40R20?
305/40R20 is outside the ±3% rule as a replacement for 305/35R20: the diameter differs by +4.23%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 305/35R20 and 305/40R20?
305/40R20 is 1.20" taller and 0 mm wider; the sidewall is +0.60". At an indicated 60 mph you'd actually be doing 62.5 mph.
Will 305/40R20 fit on 305/35R20 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.