275/40R21 vs 275/45R21
275/45R21 is outside the ±3% rule as a replacement for 275/40R21: the diameter differs by +3.65%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 275/45R21 on a car calibrated for 275/40R21, the speedometer reads 57.9 mph when you're really doing 60, and the odometer under-counts by 3.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.1 mph |
| 45 mph | 46.6 mph |
| 60 mph | 62.2 mph |
| 70 mph | 72.6 mph |
| 80 mph | 82.9 mph |
Other sizes close to 275/40R21
- 275/40R21 vs 245/50R20-0.05%
- 275/40R21 vs 275/45R20+0.28%
- 275/40R21 vs 265/45R20-0.92%
- 275/40R21 vs 265/40R21-1.06%
- 275/40R21 vs 275/35R21-3.65%
- 275/40R21 vs 295/35R21-1.79%
- 275/40R21 vs 255/40R21-2.12%
- 275/40R21 vs 265/35R21-4.58%
- 275/40R21 vs 285/35R22+0.65%
- 275/40R21 vs 295/40R21+2.12%
- 275/40R21 vs 285/45R21+4.84%
- 275/40R21 vs 285/40R21+1.06%
Questions
Can I replace 275/40R21 with 275/45R21?
275/45R21 is outside the ±3% rule as a replacement for 275/40R21: the diameter differs by +3.65%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 275/40R21 and 275/45R21?
275/45R21 is 1.08" taller and 0 mm wider; the sidewall is +0.54". At an indicated 60 mph you'd actually be doing 62.2 mph.
Will 275/45R21 fit on 275/40R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 275 mm tire and the fender clearance.