255/35R21 vs 255/40R21
255/40R21 is outside the ±3% rule as a replacement for 255/35R21: the diameter differs by +3.58%, 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 255/35R21, the speedometer reads 57.9 mph when you're really doing 60, and the odometer under-counts by 3.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.1 mph |
| 45 mph | 46.6 mph |
| 60 mph | 62.1 mph |
| 70 mph | 72.5 mph |
| 80 mph | 82.9 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 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%
- 255/35R21 vs 245/40R21+2.46%
Questions
Can I replace 255/35R21 with 255/40R21?
255/40R21 is outside the ±3% rule as a replacement for 255/35R21: the diameter differs by +3.58%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 255/35R21 and 255/40R21?
255/40R21 is 1.00" taller and 0 mm wider; the sidewall is +0.50". At an indicated 60 mph you'd actually be doing 62.1 mph.
Will 255/40R21 fit on 255/35R21 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.