255/35R21 vs 245/40R21
245/40R21 is a safe swap for 255/35R21 by the usual ±3% rule: the diameter differs by +2.46%.
Speedometer and odometer
With 245/40R21 on a car calibrated for 255/35R21, the speedometer reads 58.6 mph when you're really doing 60, and the odometer under-counts by 2.5%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.7 mph |
| 45 mph | 46.1 mph |
| 60 mph | 61.5 mph |
| 70 mph | 71.7 mph |
| 80 mph | 82.0 mph |
Other sizes close to 255/35R21
- 255/35R21 vs 255/40R20+0.01%
- 255/35R21 vs 285/35R20-0.62%
- 255/35R21 vs 265/40R21+4.71%
- 255/35R21 vs 275/35R21+1.97%
- 255/35R21 vs 295/35R21+3.93%
- 255/35R21 vs 255/40R21+3.58%
- 255/35R21 vs 265/35R21+0.98%
- 255/35R21 vs 295/35R20+0.37%
- 255/35R21 vs 245/35R21-0.98%
- 255/35R21 vs 325/30R21+2.32%
- 255/35R21 vs 315/30R21+1.47%
- 255/35R21 vs 285/35R21+2.95%
Questions
Can I replace 255/35R21 with 245/40R21?
245/40R21 is a safe swap for 255/35R21 by the usual ±3% rule: the diameter differs by +2.46%.
What's the difference between 255/35R21 and 245/40R21?
245/40R21 is 0.69" taller and 10 mm narrower; the sidewall is +0.34". At an indicated 60 mph you'd actually be doing 61.5 mph.
Will 245/40R21 fit on 255/35R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 245 mm tire and the fender clearance.