275/40R21 vs 255/45R21
255/45R21 is a safe swap for 275/40R21 by the usual ±3% rule: the diameter differs by +1.26%.
Speedometer and odometer
With 255/45R21 on a car calibrated for 275/40R21, the speedometer reads 59.3 mph when you're really doing 60, and the odometer under-counts by 1.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.4 mph |
| 45 mph | 45.6 mph |
| 60 mph | 60.8 mph |
| 70 mph | 70.9 mph |
| 80 mph | 81.0 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 275/45R21+3.65%
- 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%
Questions
Can I replace 275/40R21 with 255/45R21?
255/45R21 is a safe swap for 275/40R21 by the usual ±3% rule: the diameter differs by +1.26%.
What's the difference between 275/40R21 and 255/45R21?
255/45R21 is 0.37" taller and 20 mm narrower; the sidewall is +0.19". At an indicated 60 mph you'd actually be doing 60.8 mph.
Will 255/45R21 fit on 275/40R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.