285/30R21 vs 305/30R21
305/30R21 is a safe swap for 285/30R21 by the usual ±3% rule: the diameter differs by +1.70%.
Speedometer and odometer
With 305/30R21 on a car calibrated for 285/30R21, the speedometer reads 59.0 mph when you're really doing 60, and the odometer under-counts by 1.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.5 mph |
| 45 mph | 45.8 mph |
| 60 mph | 61.0 mph |
| 70 mph | 71.2 mph |
| 80 mph | 81.4 mph |
Other sizes close to 285/30R21
- 285/30R21 vs 245/40R20-0.06%
- 285/30R21 vs 275/35R20-0.55%
- 285/30R21 vs 285/35R20+0.44%
- 285/30R21 vs 275/35R21+3.05%
- 285/30R21 vs 255/40R21+4.68%
- 285/30R21 vs 265/35R21+2.06%
- 285/30R21 vs 245/35R21+0.07%
- 285/30R21 vs 325/30R21+3.41%
- 285/30R21 vs 255/35R21+1.06%
- 285/30R21 vs 315/30R21+2.56%
- 285/30R21 vs 285/35R21+4.05%
- 285/30R21 vs 245/40R21+3.55%
Questions
Can I replace 285/30R21 with 305/30R21?
305/30R21 is a safe swap for 285/30R21 by the usual ±3% rule: the diameter differs by +1.70%.
What's the difference between 285/30R21 and 305/30R21?
305/30R21 is 0.47" taller and 20 mm wider; the sidewall is +0.24". At an indicated 60 mph you'd actually be doing 61.0 mph.
Will 305/30R21 fit on 285/30R21 wheels?
Both use a 21" rim, so the wheel diameter matches; check the wheel's width range for a 305 mm tire and the fender clearance.