245/40R20 vs 285/30R21
285/30R21 is a safe swap for 245/40R20 by the usual ±3% rule: the diameter differs by +0.06%.
Speedometer and odometer
With 285/30R21 on a car calibrated for 245/40R20, the speedometer reads 60.0 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 | 45.0 mph |
| 60 mph | 60.0 mph |
| 70 mph | 70.0 mph |
| 80 mph | 80.0 mph |
Other sizes close to 245/40R20
- 245/40R20 vs 245/45R19-0.13%
- 245/40R20 vs 245/45R20+3.48%
- 245/40R20 vs 255/45R20+4.76%
- 245/40R20 vs 275/40R20+3.41%
- 245/40R20 vs 275/40R19-0.20%
- 245/40R20 vs 245/35R20-3.48%
- 245/40R20 vs 255/35R20-2.49%
- 245/40R20 vs 275/35R20-0.50%
- 245/40R20 vs 255/40R20+1.14%
- 245/40R20 vs 285/35R20+0.50%
- 245/40R20 vs 315/35R20+3.48%
- 245/40R20 vs 285/30R20-3.55%
Questions
Can I replace 245/40R20 with 285/30R21?
285/30R21 is a safe swap for 245/40R20 by the usual ±3% rule: the diameter differs by +0.06%.
What's the difference between 245/40R20 and 285/30R21?
285/30R21 is 0.02" taller and 40 mm wider; the sidewall is -0.49". At an indicated 60 mph you'd actually be doing 60.0 mph.
Will 285/30R21 fit on 245/40R20 wheels?
No — 245/40R20 needs a 20" wheel and 285/30R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.