255/40R21 vs 235/50R20
235/50R20 is a safe swap for 255/40R21 by the usual ±3% rule: the diameter differs by +0.76%.
Speedometer and odometer
With 235/50R20 on a car calibrated for 255/40R21, the speedometer reads 59.5 mph when you're really doing 60, and the odometer under-counts by 0.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.5 mph |
| 70 mph | 70.5 mph |
| 80 mph | 80.6 mph |
Other sizes close to 255/40R21
- 255/40R21 vs 255/45R20+0.01%
- 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%
Questions
Can I replace 255/40R21 with 235/50R20?
235/50R20 is a safe swap for 255/40R21 by the usual ±3% rule: the diameter differs by +0.76%.
What's the difference between 255/40R21 and 235/50R20?
235/50R20 is 0.22" taller and 20 mm narrower; the sidewall is +0.61". At an indicated 60 mph you'd actually be doing 60.5 mph.
Will 235/50R20 fit on 255/40R21 wheels?
No — 255/40R21 needs a 21" wheel and 235/50R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.