275/45R20 vs 285/40R21
285/40R21 is a safe swap for 275/45R20 by the usual ±3% rule: the diameter differs by +0.78%.
Speedometer and odometer
With 285/40R21 on a car calibrated for 275/45R20, the speedometer reads 59.5 mph when you're really doing 60, and the odometer under-counts by 0.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.4 mph |
| 60 mph | 60.5 mph |
| 70 mph | 70.5 mph |
| 80 mph | 80.6 mph |
Other sizes close to 275/45R20
- 275/45R20 vs 245/45R20-3.57%
- 275/45R20 vs 255/45R20-2.38%
- 275/45R20 vs 255/50R20+0.99%
- 275/45R20 vs 275/40R20-3.64%
- 275/45R20 vs 265/50R20+2.32%
- 275/45R20 vs 245/50R20-0.33%
- 275/45R20 vs 255/55R20+4.37%
- 275/45R20 vs 235/55R20+1.46%
- 275/45R20 vs 245/55R19-0.45%
- 275/45R20 vs 265/45R20-1.19%
- 275/45R20 vs 315/35R20-3.57%
- 275/45R20 vs 275/50R20+3.64%
Questions
Can I replace 275/45R20 with 285/40R21?
285/40R21 is a safe swap for 275/45R20 by the usual ±3% rule: the diameter differs by +0.78%.
What's the difference between 275/45R20 and 285/40R21?
285/40R21 is 0.23" taller and 10 mm wider; the sidewall is -0.38". At an indicated 60 mph you'd actually be doing 60.5 mph.
Will 285/40R21 fit on 275/45R20 wheels?
No — 275/45R20 needs a 20" wheel and 285/40R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.