305/30R21 vs 305/35R20
305/35R20 is a safe swap for 305/30R21 by the usual ±3% rule: the diameter differs by +0.71%.
Speedometer and odometer
With 305/35R20 on a car calibrated for 305/30R21, the speedometer reads 59.6 mph when you're really doing 60, and the odometer under-counts by 0.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.4 mph |
| 70 mph | 70.5 mph |
| 80 mph | 80.6 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 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 305/35R20?
305/35R20 is a safe swap for 305/30R21 by the usual ±3% rule: the diameter differs by +0.71%.
What's the difference between 305/30R21 and 305/35R20?
305/35R20 is 0.20" taller and 0 mm wider; the sidewall is +0.60". At an indicated 60 mph you'd actually be doing 60.4 mph.
Will 305/35R20 fit on 305/30R21 wheels?
No — 305/30R21 needs a 21" wheel and 305/35R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.