285/30R21 vs 255/40R21
255/40R21 is outside the ±3% rule as a replacement for 285/30R21: the diameter differs by +4.68%, 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 285/30R21, the speedometer reads 57.3 mph when you're really doing 60, and the odometer under-counts by 4.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.4 mph |
| 45 mph | 47.1 mph |
| 60 mph | 62.8 mph |
| 70 mph | 73.3 mph |
| 80 mph | 83.7 mph |
Other sizes close to 285/30R21
- 285/30R21 vs 245/40R20-0.06%
- 285/30R21 vs 275/35R20-0.55%
- 285/30R21 vs 285/35R20+0.44%
- 285/30R21 vs 275/35R21+3.05%
- 285/30R21 vs 265/35R21+2.06%
- 285/30R21 vs 245/35R21+0.07%
- 285/30R21 vs 325/30R21+3.41%
- 285/30R21 vs 255/35R21+1.06%
- 285/30R21 vs 315/30R21+2.56%
- 285/30R21 vs 285/35R21+4.05%
- 285/30R21 vs 245/40R21+3.55%
- 285/30R21 vs 295/30R21+0.85%
Questions
Can I replace 285/30R21 with 255/40R21?
255/40R21 is outside the ±3% rule as a replacement for 285/30R21: the diameter differs by +4.68%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 285/30R21 and 255/40R21?
255/40R21 is 1.30" taller and 30 mm narrower; the sidewall is +0.65". At an indicated 60 mph you'd actually be doing 62.8 mph.
Will 255/40R21 fit on 285/30R21 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.