255/40R21 vs 255/45R20
255/45R20 is a safe swap for 255/40R21 by the usual ±3% rule: the diameter differs by +0.01%.
Speedometer and odometer
With 255/45R20 on a car calibrated for 255/40R21, the speedometer reads 60.0 mph when you're really doing 60, and the odometer under-counts by 0.0%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.0 mph |
| 45 mph | 45.0 mph |
| 60 mph | 60.0 mph |
| 70 mph | 70.0 mph |
| 80 mph | 80.0 mph |
Other sizes close to 255/40R21
- 255/40R21 vs 265/40R21+1.08%
- 255/40R21 vs 275/35R21-1.56%
- 255/40R21 vs 295/35R21+0.34%
- 255/40R21 vs 265/35R21-2.51%
- 255/40R21 vs 265/35R22+0.94%
- 255/40R21 vs 295/40R20+0.90%
- 255/40R21 vs 295/40R21+4.34%
- 255/40R21 vs 245/35R21-4.41%
- 255/40R21 vs 285/40R21+3.25%
- 255/40R21 vs 265/45R21+4.68%
- 255/40R21 vs 285/40R20-0.19%
- 255/40R21 vs 325/30R21-1.22%
Questions
Can I replace 255/40R21 with 255/45R20?
255/45R20 is a safe swap for 255/40R21 by the usual ±3% rule: the diameter differs by +0.01%.
What's the difference between 255/40R21 and 255/45R20?
255/45R20 is 0.00" taller and 0 mm wider; the sidewall is +0.50". At an indicated 60 mph you'd actually be doing 60.0 mph.
Will 255/45R20 fit on 255/40R21 wheels?
No — 255/40R21 needs a 21" wheel and 255/45R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.