285/35R21 vs 255/40R21
255/40R21 is a safe swap for 285/35R21 by the usual ±3% rule: the diameter differs by +0.61%.
Speedometer and odometer
With 255/40R21 on a car calibrated for 285/35R21, the speedometer reads 59.6 mph when you're really doing 60, and the odometer under-counts by 0.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.4 mph |
| 70 mph | 70.4 mph |
| 80 mph | 80.5 mph |
Other sizes close to 285/35R21
- 285/35R21 vs 245/45R20-0.60%
- 285/35R21 vs 255/45R20+0.63%
- 285/35R21 vs 275/40R20-0.67%
- 285/35R21 vs 265/40R21+1.71%
- 285/35R21 vs 315/35R20-0.60%
- 285/35R21 vs 275/35R21-0.96%
- 285/35R21 vs 295/35R21+0.96%
- 285/35R21 vs 265/35R21-1.91%
- 285/35R21 vs 245/35R21-3.82%
- 285/35R21 vs 295/40R21+4.98%
- 285/35R21 vs 285/40R21+3.89%
- 285/35R21 vs 285/40R20+0.42%
Questions
Can I replace 285/35R21 with 255/40R21?
255/40R21 is a safe swap for 285/35R21 by the usual ±3% rule: the diameter differs by +0.61%.
What's the difference between 285/35R21 and 255/40R21?
255/40R21 is 0.18" taller and 30 mm narrower; the sidewall is +0.09". At an indicated 60 mph you'd actually be doing 60.4 mph.
Will 255/40R21 fit on 285/35R21 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.