325/30R21 vs 285/40R21
285/40R21 is outside the ±3% rule as a replacement for 325/30R21: the diameter differs by +4.53%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 285/40R21 on a car calibrated for 325/30R21, the speedometer reads 57.4 mph when you're really doing 60, and the odometer under-counts by 4.5%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.4 mph |
| 45 mph | 47.0 mph |
| 60 mph | 62.7 mph |
| 70 mph | 73.2 mph |
| 80 mph | 83.6 mph |
Other sizes close to 325/30R21
- 325/30R21 vs 245/45R20+0.01%
- 325/30R21 vs 275/40R20-0.05%
- 325/30R21 vs 265/40R21+2.33%
- 325/30R21 vs 315/35R20+0.01%
- 325/30R21 vs 275/35R21-0.34%
- 325/30R21 vs 295/35R21+1.58%
- 325/30R21 vs 255/40R21+1.24%
- 325/30R21 vs 265/35R21-1.30%
- 325/30R21 vs 245/35R21-3.23%
- 325/30R21 vs 305/35R20-0.95%
- 325/30R21 vs 275/40R21+3.43%
- 325/30R21 vs 255/35R21-2.27%
Questions
Can I replace 325/30R21 with 285/40R21?
285/40R21 is outside the ±3% rule as a replacement for 325/30R21: the diameter differs by +4.53%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 325/30R21 and 285/40R21?
285/40R21 is 1.30" taller and 40 mm narrower; the sidewall is +0.65". At an indicated 60 mph you'd actually be doing 62.7 mph.
Will 285/40R21 fit on 325/30R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 285 mm tire and the fender clearance.