325/30R21 vs 315/35R20
315/35R20 is a safe swap for 325/30R21 by the usual ±3% rule: the diameter differs by +0.01%.
Speedometer and odometer
With 315/35R20 on a car calibrated for 325/30R21, the speedometer reads 60.0 mph when you're really doing 60, and the odometer under-counts by 0.0%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.0 mph |
| 45 mph | 45.0 mph |
| 60 mph | 60.0 mph |
| 70 mph | 70.0 mph |
| 80 mph | 80.0 mph |
Other sizes close to 325/30R21
- 325/30R21 vs 245/45R20+0.01%
- 325/30R21 vs 275/40R20-0.05%
- 325/30R21 vs 265/40R21+2.33%
- 325/30R21 vs 275/35R21-0.34%
- 325/30R21 vs 295/35R21+1.58%
- 325/30R21 vs 255/40R21+1.24%
- 325/30R21 vs 265/35R21-1.30%
- 325/30R21 vs 245/35R21-3.23%
- 325/30R21 vs 305/35R20-0.95%
- 325/30R21 vs 285/40R21+4.53%
- 325/30R21 vs 275/40R21+3.43%
- 325/30R21 vs 255/35R21-2.27%
Questions
Can I replace 325/30R21 with 315/35R20?
315/35R20 is a safe swap for 325/30R21 by the usual ±3% rule: the diameter differs by +0.01%.
What's the difference between 325/30R21 and 315/35R20?
315/35R20 is 0.00" taller and 10 mm narrower; the sidewall is +0.50". At an indicated 60 mph you'd actually be doing 60.0 mph.
Will 315/35R20 fit on 325/30R21 wheels?
No — 325/30R21 needs a 21" wheel and 315/35R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.