295/30R21 vs 255/40R21
255/40R21 is outside the ±3% rule as a replacement for 295/30R21: the diameter differs by +3.80%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/40R21 on a car calibrated for 295/30R21, the speedometer reads 57.8 mph when you're really doing 60, and the odometer under-counts by 3.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.1 mph |
| 45 mph | 46.7 mph |
| 60 mph | 62.3 mph |
| 70 mph | 72.7 mph |
| 80 mph | 83.0 mph |
Other sizes close to 295/30R21
- 295/30R21 vs 245/40R20-0.90%
- 295/30R21 vs 255/40R20+0.23%
- 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 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 255/40R21?
255/40R21 is outside the ±3% rule as a replacement for 295/30R21: the diameter differs by +3.80%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 295/30R21 and 255/40R21?
255/40R21 is 1.06" taller and 40 mm narrower; the sidewall is +0.53". At an indicated 60 mph you'd actually be doing 62.3 mph.
Will 255/40R21 fit on 295/30R21 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.