255/45R21 vs 235/55R20
235/55R20 is a safe swap for 255/45R21 by the usual ±3% rule: the diameter differs by +0.47%.
Speedometer and odometer
With 235/55R20 on a car calibrated for 255/45R21, the speedometer reads 59.7 mph when you're really doing 60, and the odometer under-counts by 0.5%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.2 mph |
| 60 mph | 60.3 mph |
| 70 mph | 70.3 mph |
| 80 mph | 80.4 mph |
Other sizes close to 255/45R21
- 255/45R21 vs 255/50R20+0.01%
- 255/45R21 vs 275/45R20-0.97%
- 255/45R21 vs 265/40R21-2.29%
- 255/45R21 vs 275/35R21-4.85%
- 255/45R21 vs 295/35R21-3.01%
- 255/45R21 vs 275/45R21+2.36%
- 255/45R21 vs 255/40R21-3.34%
- 255/45R21 vs 285/35R22-0.60%
- 255/45R21 vs 295/40R21+0.85%
- 255/45R21 vs 285/45R21+3.54%
- 255/45R21 vs 285/40R21-0.20%
- 255/45R21 vs 265/45R21+1.18%
Questions
Can I replace 255/45R21 with 235/55R20?
235/55R20 is a safe swap for 255/45R21 by the usual ±3% rule: the diameter differs by +0.47%.
What's the difference between 255/45R21 and 235/55R20?
235/55R20 is 0.14" taller and 20 mm narrower; the sidewall is +0.57". At an indicated 60 mph you'd actually be doing 60.3 mph.
Will 235/55R20 fit on 255/45R21 wheels?
No — 255/45R21 needs a 21" wheel and 235/55R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.