315/30R21 vs 275/40R21
275/40R21 is outside the ±3% rule as a replacement for 315/30R21: the diameter differs by +4.29%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 275/40R21 on a car calibrated for 315/30R21, the speedometer reads 57.5 mph when you're really doing 60, and the odometer under-counts by 4.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.3 mph |
| 45 mph | 46.9 mph |
| 60 mph | 62.6 mph |
| 70 mph | 73.0 mph |
| 80 mph | 83.4 mph |
Other sizes close to 315/30R21
- 315/30R21 vs 245/45R20+0.84%
- 315/30R21 vs 275/40R20+0.78%
- 315/30R21 vs 265/40R21+3.18%
- 315/30R21 vs 315/35R20+0.84%
- 315/30R21 vs 275/35R21+0.48%
- 315/30R21 vs 295/35R21+2.42%
- 315/30R21 vs 255/40R21+2.08%
- 315/30R21 vs 265/35R21-0.48%
- 315/30R21 vs 245/35R21-2.42%
- 315/30R21 vs 305/35R20-0.12%
- 315/30R21 vs 325/30R21+0.83%
- 315/30R21 vs 235/45R20-0.40%
Questions
Can I replace 315/30R21 with 275/40R21?
275/40R21 is outside the ±3% rule as a replacement for 315/30R21: the diameter differs by +4.29%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 315/30R21 and 275/40R21?
275/40R21 is 1.22" taller and 40 mm narrower; the sidewall is +0.61". At an indicated 60 mph you'd actually be doing 62.6 mph.
Will 275/40R21 fit on 315/30R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 275 mm tire and the fender clearance.