255/40R20 vs 305/30R21
305/30R21 is a safe swap for 255/40R20 by the usual ±3% rule: the diameter differs by +0.62%.
Speedometer and odometer
With 305/30R21 on a car calibrated for 255/40R20, the speedometer reads 59.6 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 | 60.4 mph |
| 70 mph | 70.4 mph |
| 80 mph | 80.5 mph |
Other sizes close to 255/40R20
- 255/40R20 vs 245/45R20+2.32%
- 255/40R20 vs 255/45R20+3.58%
- 255/40R20 vs 275/40R20+2.25%
- 255/40R20 vs 245/40R20-1.12%
- 255/40R20 vs 255/45R19+0.01%
- 255/40R20 vs 245/35R20-4.56%
- 255/40R20 vs 255/35R20-3.58%
- 255/40R20 vs 275/35R20-1.62%
- 255/40R20 vs 235/50R19+0.79%
- 255/40R20 vs 265/45R20+4.85%
- 255/40R20 vs 285/35R20-0.63%
- 255/40R20 vs 315/35R20+2.32%
Questions
Can I replace 255/40R20 with 305/30R21?
305/30R21 is a safe swap for 255/40R20 by the usual ±3% rule: the diameter differs by +0.62%.
What's the difference between 255/40R20 and 305/30R21?
305/30R21 is 0.17" taller and 50 mm wider; the sidewall is -0.41". At an indicated 60 mph you'd actually be doing 60.4 mph.
Will 305/30R21 fit on 255/40R20 wheels?
No — 255/40R20 needs a 20" wheel and 305/30R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.