275/40R20 vs 305/40R20
305/40R20 is outside the ±3% rule as a replacement for 275/40R20: the diameter differs by +3.30%, 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 275/40R20, the speedometer reads 58.1 mph when you're really doing 60, and the odometer under-counts by 3.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.0 mph |
| 45 mph | 46.5 mph |
| 60 mph | 62.0 mph |
| 70 mph | 72.3 mph |
| 80 mph | 82.6 mph |
Other sizes close to 275/40R20
- 275/40R20 vs 245/45R20+0.07%
- 275/40R20 vs 255/45R20+1.30%
- 275/40R20 vs 255/50R20+4.81%
- 275/40R20 vs 245/40R20-3.30%
- 275/40R20 vs 245/50R20+3.43%
- 275/40R20 vs 275/45R20+3.78%
- 275/40R20 vs 225/55R19+0.29%
- 275/40R20 vs 255/40R20-2.20%
- 275/40R20 vs 275/35R20-3.78%
- 275/40R20 vs 265/45R20+2.54%
- 275/40R20 vs 285/35R20-2.82%
- 275/40R20 vs 315/35R20+0.07%
Questions
Can I replace 275/40R20 with 305/40R20?
305/40R20 is outside the ±3% rule as a replacement for 275/40R20: the diameter differs by +3.30%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 275/40R20 and 305/40R20?
305/40R20 is 0.94" taller and 30 mm wider; the sidewall is +0.47". At an indicated 60 mph you'd actually be doing 62.0 mph.
Will 305/40R20 fit on 275/40R20 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.