325/30R21 vs 255/40R21
255/40R21 is a safe swap for 325/30R21 by the usual ±3% rule: the diameter differs by +1.24%.
Speedometer and odometer
With 255/40R21 on a car calibrated for 325/30R21, the speedometer reads 59.3 mph when you're really doing 60, and the odometer under-counts by 1.2%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.4 mph |
| 45 mph | 45.6 mph |
| 60 mph | 60.7 mph |
| 70 mph | 70.9 mph |
| 80 mph | 81.0 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 265/35R21-1.30%
- 325/30R21 vs 245/35R21-3.23%
- 325/30R21 vs 305/35R20-0.95%
- 325/30R21 vs 285/40R21+4.53%
- 325/30R21 vs 275/40R21+3.43%
- 325/30R21 vs 255/35R21-2.27%
Questions
Can I replace 325/30R21 with 255/40R21?
255/40R21 is a safe swap for 325/30R21 by the usual ±3% rule: the diameter differs by +1.24%.
What's the difference between 325/30R21 and 255/40R21?
255/40R21 is 0.35" taller and 70 mm narrower; the sidewall is +0.18". At an indicated 60 mph you'd actually be doing 60.7 mph.
Will 255/40R21 fit on 325/30R21 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.