305/35R20 vs 315/30R21
315/30R21 is a safe swap for 305/35R20 by the usual ±3% rule: the diameter differs by +0.12%.
Speedometer and odometer
With 315/30R21 on a car calibrated for 305/35R20, the speedometer reads 59.9 mph when you're really doing 60, and the odometer under-counts by 0.1%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.0 mph |
| 45 mph | 45.1 mph |
| 60 mph | 60.1 mph |
| 70 mph | 70.1 mph |
| 80 mph | 80.1 mph |
Other sizes close to 305/35R20
- 305/35R20 vs 245/45R20+0.97%
- 305/35R20 vs 255/45R20+2.22%
- 305/35R20 vs 275/40R20+0.90%
- 305/35R20 vs 245/40R20-2.43%
- 305/35R20 vs 245/50R20+4.37%
- 305/35R20 vs 275/45R20+4.71%
- 305/35R20 vs 255/35R20-4.85%
- 305/35R20 vs 255/40R20-1.32%
- 305/35R20 vs 275/35R20-2.91%
- 305/35R20 vs 235/50R19-0.54%
- 305/35R20 vs 265/45R20+3.47%
- 305/35R20 vs 285/35R20-1.94%
Questions
Can I replace 305/35R20 with 315/30R21?
315/30R21 is a safe swap for 305/35R20 by the usual ±3% rule: the diameter differs by +0.12%.
What's the difference between 305/35R20 and 315/30R21?
315/30R21 is 0.04" taller and 10 mm wider; the sidewall is -0.48". At an indicated 60 mph you'd actually be doing 60.1 mph.
Will 315/30R21 fit on 305/35R20 wheels?
No — 305/35R20 needs a 20" wheel and 315/30R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.