265/40R21 vs 305/40R20
305/40R20 is a safe swap for 265/40R21 by the usual ±3% rule: the diameter differs by +0.89%.
Speedometer and odometer
With 305/40R20 on a car calibrated for 265/40R21, the speedometer reads 59.5 mph when you're really doing 60, and the odometer under-counts by 0.9%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.3 mph |
| 45 mph | 45.4 mph |
| 60 mph | 60.5 mph |
| 70 mph | 70.6 mph |
| 80 mph | 80.7 mph |
Other sizes close to 265/40R21
- 265/40R21 vs 265/45R20+0.15%
- 265/40R21 vs 275/35R21-2.62%
- 265/40R21 vs 295/35R21-0.74%
- 265/40R21 vs 275/45R21+4.76%
- 265/40R21 vs 255/40R21-1.07%
- 265/40R21 vs 265/35R21-3.56%
- 265/40R21 vs 265/35R22-0.15%
- 265/40R21 vs 295/40R20-0.19%
- 265/40R21 vs 295/40R21+3.22%
- 265/40R21 vs 285/40R21+2.15%
- 265/40R21 vs 265/45R21+3.56%
- 265/40R21 vs 325/30R21-2.28%
Questions
Can I replace 265/40R21 with 305/40R20?
305/40R20 is a safe swap for 265/40R21 by the usual ±3% rule: the diameter differs by +0.89%.
What's the difference between 265/40R21 and 305/40R20?
305/40R20 is 0.26" taller and 40 mm wider; the sidewall is +0.63". At an indicated 60 mph you'd actually be doing 60.5 mph.
Will 305/40R20 fit on 265/40R21 wheels?
No — 265/40R21 needs a 21" wheel and 305/40R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.