275/40R21 vs 275/45R20
275/45R20 is a safe swap for 275/40R21 by the usual ±3% rule: the diameter differs by +0.28%.
Speedometer and odometer
With 275/45R20 on a car calibrated for 275/40R21, the speedometer reads 59.8 mph when you're really doing 60, and the odometer under-counts by 0.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.1 mph |
| 60 mph | 60.2 mph |
| 70 mph | 70.2 mph |
| 80 mph | 80.2 mph |
Other sizes close to 275/40R21
- 275/40R21 vs 245/50R20-0.05%
- 275/40R21 vs 265/45R20-0.92%
- 275/40R21 vs 265/40R21-1.06%
- 275/40R21 vs 275/35R21-3.65%
- 275/40R21 vs 295/35R21-1.79%
- 275/40R21 vs 275/45R21+3.65%
- 275/40R21 vs 255/40R21-2.12%
- 275/40R21 vs 265/35R21-4.58%
- 275/40R21 vs 285/35R22+0.65%
- 275/40R21 vs 295/40R21+2.12%
- 275/40R21 vs 285/45R21+4.84%
- 275/40R21 vs 285/40R21+1.06%
Questions
Can I replace 275/40R21 with 275/45R20?
275/45R20 is a safe swap for 275/40R21 by the usual ±3% rule: the diameter differs by +0.28%.
What's the difference between 275/40R21 and 275/45R20?
275/45R20 is 0.08" taller and 0 mm wider; the sidewall is +0.54". At an indicated 60 mph you'd actually be doing 60.2 mph.
Will 275/45R20 fit on 275/40R21 wheels?
No — 275/40R21 needs a 21" wheel and 275/45R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.