275/30R21 vs 245/40R21
245/40R21 is outside the ±3% rule as a replacement for 275/30R21: the diameter differs by +4.44%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 245/40R21 on a car calibrated for 275/30R21, the speedometer reads 57.4 mph when you're really doing 60, and the odometer under-counts by 4.4%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.3 mph |
| 45 mph | 47.0 mph |
| 60 mph | 62.7 mph |
| 70 mph | 73.1 mph |
| 80 mph | 83.6 mph |
Other sizes close to 275/30R21
- 275/30R21 vs 245/40R20+0.80%
- 275/30R21 vs 275/35R20+0.30%
- 275/30R21 vs 265/35R20-0.70%
- 275/30R21 vs 275/35R21+3.94%
- 275/30R21 vs 265/35R21+2.94%
- 275/30R21 vs 245/35R21+0.93%
- 275/30R21 vs 325/30R21+4.30%
- 275/30R21 vs 255/35R21+1.93%
- 275/30R21 vs 315/30R21+3.44%
- 275/30R21 vs 285/35R21+4.94%
- 275/30R21 vs 295/30R21+1.72%
- 275/30R21 vs 305/30R21+2.58%
Questions
Can I replace 275/30R21 with 245/40R21?
245/40R21 is outside the ±3% rule as a replacement for 275/30R21: the diameter differs by +4.44%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 275/30R21 and 245/40R21?
245/40R21 is 1.22" taller and 30 mm narrower; the sidewall is +0.61". At an indicated 60 mph you'd actually be doing 62.7 mph.
Will 245/40R21 fit on 275/30R21 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.