305/30R21 vs 255/40R21
255/40R21 is a safe swap for 305/30R21 by the usual ±3% rule: the diameter differs by +2.93%.
Speedometer and odometer
With 255/40R21 on a car calibrated for 305/30R21, the speedometer reads 58.3 mph when you're really doing 60, and the odometer under-counts by 2.9%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.9 mph |
| 45 mph | 46.3 mph |
| 60 mph | 61.8 mph |
| 70 mph | 72.1 mph |
| 80 mph | 82.3 mph |
Other sizes close to 305/30R21
- 305/30R21 vs 255/40R20-0.61%
- 305/30R21 vs 265/40R21+4.05%
- 305/30R21 vs 275/35R21+1.33%
- 305/30R21 vs 295/35R21+3.28%
- 305/30R21 vs 265/35R21+0.35%
- 305/30R21 vs 295/35R20-0.27%
- 305/30R21 vs 245/35R21-1.61%
- 305/30R21 vs 305/35R20+0.71%
- 305/30R21 vs 325/30R21+1.68%
- 305/30R21 vs 235/45R20+0.43%
- 305/30R21 vs 255/35R21-0.63%
- 305/30R21 vs 315/30R21+0.84%
Questions
Can I replace 305/30R21 with 255/40R21?
255/40R21 is a safe swap for 305/30R21 by the usual ±3% rule: the diameter differs by +2.93%.
What's the difference between 305/30R21 and 255/40R21?
255/40R21 is 0.83" taller and 50 mm narrower; the sidewall is +0.41". At an indicated 60 mph you'd actually be doing 61.8 mph.
Will 255/40R21 fit on 305/30R21 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.