235/45R20 vs 315/30R21
315/30R21 is a safe swap for 235/45R20 by the usual ±3% rule: the diameter differs by +0.40%.
Speedometer and odometer
With 315/30R21 on a car calibrated for 235/45R20, the speedometer reads 59.8 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 | 60.2 mph |
| 70 mph | 70.3 mph |
| 80 mph | 80.3 mph |
Other sizes close to 235/45R20
- 235/45R20 vs 245/45R20+1.25%
- 235/45R20 vs 255/45R20+2.50%
- 235/45R20 vs 275/40R20+1.18%
- 235/45R20 vs 245/40R20-2.15%
- 235/45R20 vs 245/50R20+4.66%
- 235/45R20 vs 255/35R20-4.59%
- 235/45R20 vs 255/40R20-1.04%
- 235/45R20 vs 275/35R20-2.64%
- 235/45R20 vs 235/50R19-0.26%
- 235/45R20 vs 265/45R20+3.75%
- 235/45R20 vs 285/35R20-1.67%
- 235/45R20 vs 315/35R20+1.25%
Questions
Can I replace 235/45R20 with 315/30R21?
315/30R21 is a safe swap for 235/45R20 by the usual ±3% rule: the diameter differs by +0.40%.
What's the difference between 235/45R20 and 315/30R21?
315/30R21 is 0.11" taller and 80 mm wider; the sidewall is -0.44". At an indicated 60 mph you'd actually be doing 60.2 mph.
Will 315/30R21 fit on 235/45R20 wheels?
No — 235/45R20 needs a 20" wheel and 315/30R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.