275/35R21 vs 255/40R21
255/40R21 is a safe swap for 275/35R21 by the usual ±3% rule: the diameter differs by +1.58%.
Speedometer and odometer
With 255/40R21 on a car calibrated for 275/35R21, the speedometer reads 59.1 mph when you're really doing 60, and the odometer under-counts by 1.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.5 mph |
| 45 mph | 45.7 mph |
| 60 mph | 61.0 mph |
| 70 mph | 71.1 mph |
| 80 mph | 81.3 mph |
Other sizes close to 275/35R21
- 275/35R21 vs 245/45R20+0.36%
- 275/35R21 vs 275/40R20+0.29%
- 275/35R21 vs 265/40R21+2.69%
- 275/35R21 vs 315/35R20+0.36%
- 275/35R21 vs 295/35R21+1.93%
- 275/35R21 vs 265/35R21-0.96%
- 275/35R21 vs 245/35R21-2.89%
- 275/35R21 vs 305/35R20-0.61%
- 275/35R21 vs 285/40R21+4.89%
- 275/35R21 vs 325/30R21+0.34%
- 275/35R21 vs 275/40R21+3.79%
- 275/35R21 vs 235/45R20-0.88%
Questions
Can I replace 275/35R21 with 255/40R21?
255/40R21 is a safe swap for 275/35R21 by the usual ±3% rule: the diameter differs by +1.58%.
What's the difference between 275/35R21 and 255/40R21?
255/40R21 is 0.45" taller and 20 mm narrower; the sidewall is +0.23". At an indicated 60 mph you'd actually be doing 61.0 mph.
Will 255/40R21 fit on 275/35R21 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.