275/50R21 vs 305/50R20
305/50R20 is a safe swap for 275/50R21 by the usual ±3% rule: the diameter differs by +0.57%.
Speedometer and odometer
With 305/50R20 on a car calibrated for 275/50R21, the speedometer reads 59.7 mph when you're really doing 60, and the odometer under-counts by 0.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.3 mph |
| 60 mph | 60.3 mph |
| 70 mph | 70.4 mph |
| 80 mph | 80.5 mph |
Other sizes close to 275/50R21
- 275/50R21 vs 275/55R20+0.26%
- 275/50R21 vs 285/45R22+0.85%
- 275/50R21 vs 275/45R21-3.40%
- 275/50R21 vs 285/45R21-2.29%
- 275/50R21 vs 295/40R21-4.82%
- 275/50R21 vs 265/45R21-4.52%
- 275/50R21 vs 245/60R20-0.79%
- 275/50R21 vs 275/45R22-0.26%
- 275/50R21 vs 305/40R22-0.69%
- 275/50R21 vs 315/40R21-2.85%
- 275/50R21 vs 255/60R20+0.69%
- 275/50R21 vs 255/50R21-2.47%
Questions
Can I replace 275/50R21 with 305/50R20?
305/50R20 is a safe swap for 275/50R21 by the usual ±3% rule: the diameter differs by +0.57%.
What's the difference between 275/50R21 and 305/50R20?
305/50R20 is 0.18" taller and 30 mm wider; the sidewall is +0.59". At an indicated 60 mph you'd actually be doing 60.3 mph.
Will 305/50R20 fit on 275/50R21 wheels?
No — 275/50R21 needs a 21" wheel and 305/50R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.