285/40R21 vs 255/45R21
255/45R21 is a safe swap for 285/40R21 by the usual ±3% rule: the diameter differs by +0.20%.
Speedometer and odometer
With 255/45R21 on a car calibrated for 285/40R21, the speedometer reads 59.9 mph when you're really doing 60, and the odometer under-counts by 0.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.1 mph |
| 60 mph | 60.1 mph |
| 70 mph | 70.1 mph |
| 80 mph | 80.2 mph |
Other sizes close to 285/40R21
- 285/40R21 vs 255/50R20+0.21%
- 285/40R21 vs 275/45R20-0.77%
- 285/40R21 vs 235/55R20+0.67%
- 285/40R21 vs 265/40R21-2.10%
- 285/40R21 vs 275/35R21-4.66%
- 285/40R21 vs 295/35R21-2.82%
- 285/40R21 vs 275/45R21+2.56%
- 285/40R21 vs 255/40R21-3.15%
- 285/40R21 vs 285/35R22-0.41%
- 285/40R21 vs 295/40R21+1.05%
- 285/40R21 vs 285/45R21+3.74%
- 285/40R21 vs 265/45R21+1.38%
Questions
Can I replace 285/40R21 with 255/45R21?
255/45R21 is a safe swap for 285/40R21 by the usual ±3% rule: the diameter differs by +0.20%.
What's the difference between 285/40R21 and 255/45R21?
255/45R21 is 0.06" taller and 30 mm narrower; the sidewall is +0.03". At an indicated 60 mph you'd actually be doing 60.1 mph.
Will 255/45R21 fit on 285/40R21 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.