295/30R21 vs 255/40R20
255/40R20 is a safe swap for 295/30R21 by the usual ±3% rule: the diameter differs by +0.23%.
Speedometer and odometer
With 255/40R20 on a car calibrated for 295/30R21, 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 | 70.2 mph |
| 80 mph | 80.2 mph |
Other sizes close to 295/30R21
- 295/30R21 vs 245/40R20-0.90%
- 295/30R21 vs 285/35R20-0.41%
- 295/30R21 vs 265/40R21+4.93%
- 295/30R21 vs 275/35R21+2.18%
- 295/30R21 vs 295/35R21+4.15%
- 295/30R21 vs 255/40R21+3.80%
- 295/30R21 vs 265/35R21+1.20%
- 295/30R21 vs 295/35R20+0.58%
- 295/30R21 vs 245/35R21-0.77%
- 295/30R21 vs 325/30R21+2.53%
- 295/30R21 vs 255/35R21+0.21%
- 295/30R21 vs 315/30R21+1.69%
Questions
Can I replace 295/30R21 with 255/40R20?
255/40R20 is a safe swap for 295/30R21 by the usual ±3% rule: the diameter differs by +0.23%.
What's the difference between 295/30R21 and 255/40R20?
255/40R20 is 0.06" taller and 40 mm narrower; the sidewall is +0.53". At an indicated 60 mph you'd actually be doing 60.1 mph.
Will 255/40R20 fit on 295/30R21 wheels?
No — 295/30R21 needs a 21" wheel and 255/40R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.