305/30R21 vs 235/45R20
235/45R20 is a safe swap for 305/30R21 by the usual ±3% rule: the diameter differs by +0.43%.
Speedometer and odometer
With 235/45R20 on a car calibrated for 305/30R21, the speedometer reads 59.7 mph when you're really doing 60, and the odometer under-counts by 0.4%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.2 mph |
| 60 mph | 60.3 mph |
| 70 mph | 70.3 mph |
| 80 mph | 80.3 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 255/40R21+2.93%
- 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 255/35R21-0.63%
- 305/30R21 vs 315/30R21+0.84%
Questions
Can I replace 305/30R21 with 235/45R20?
235/45R20 is a safe swap for 305/30R21 by the usual ±3% rule: the diameter differs by +0.43%.
What's the difference between 305/30R21 and 235/45R20?
235/45R20 is 0.12" taller and 70 mm narrower; the sidewall is +0.56". At an indicated 60 mph you'd actually be doing 60.3 mph.
Will 235/45R20 fit on 305/30R21 wheels?
No — 305/30R21 needs a 21" wheel and 235/45R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.