265/40R21 vs 255/45R21
255/45R21 is a safe swap for 265/40R21 by the usual ±3% rule: the diameter differs by +2.35%.
Speedometer and odometer
With 255/45R21 on a car calibrated for 265/40R21, the speedometer reads 58.6 mph when you're really doing 60, and the odometer under-counts by 2.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.7 mph |
| 45 mph | 46.1 mph |
| 60 mph | 61.4 mph |
| 70 mph | 71.6 mph |
| 80 mph | 81.9 mph |
Other sizes close to 265/40R21
- 265/40R21 vs 265/45R20+0.15%
- 265/40R21 vs 275/35R21-2.62%
- 265/40R21 vs 295/35R21-0.74%
- 265/40R21 vs 275/45R21+4.76%
- 265/40R21 vs 255/40R21-1.07%
- 265/40R21 vs 265/35R21-3.56%
- 265/40R21 vs 265/35R22-0.15%
- 265/40R21 vs 295/40R20-0.19%
- 265/40R21 vs 295/40R21+3.22%
- 265/40R21 vs 285/40R21+2.15%
- 265/40R21 vs 265/45R21+3.56%
- 265/40R21 vs 325/30R21-2.28%
Questions
Can I replace 265/40R21 with 255/45R21?
255/45R21 is a safe swap for 265/40R21 by the usual ±3% rule: the diameter differs by +2.35%.
What's the difference between 265/40R21 and 255/45R21?
255/45R21 is 0.69" taller and 10 mm narrower; the sidewall is +0.34". At an indicated 60 mph you'd actually be doing 61.4 mph.
Will 255/45R21 fit on 265/40R21 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.