275/40R20 vs 325/30R21
325/30R21 is a safe swap for 275/40R20 by the usual ±3% rule: the diameter differs by +0.05%.
Speedometer and odometer
With 325/30R21 on a car calibrated for 275/40R20, the speedometer reads 60.0 mph when you're really doing 60, and the odometer under-counts by 0.1%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.0 mph |
| 45 mph | 45.0 mph |
| 60 mph | 60.0 mph |
| 70 mph | 70.0 mph |
| 80 mph | 80.0 mph |
Other sizes close to 275/40R20
- 275/40R20 vs 245/45R20+0.07%
- 275/40R20 vs 255/45R20+1.30%
- 275/40R20 vs 255/50R20+4.81%
- 275/40R20 vs 245/40R20-3.30%
- 275/40R20 vs 245/50R20+3.43%
- 275/40R20 vs 275/45R20+3.78%
- 275/40R20 vs 225/55R19+0.29%
- 275/40R20 vs 255/40R20-2.20%
- 275/40R20 vs 275/35R20-3.78%
- 275/40R20 vs 265/45R20+2.54%
- 275/40R20 vs 285/35R20-2.82%
- 275/40R20 vs 315/35R20+0.07%
Questions
Can I replace 275/40R20 with 325/30R21?
325/30R21 is a safe swap for 275/40R20 by the usual ±3% rule: the diameter differs by +0.05%.
What's the difference between 275/40R20 and 325/30R21?
325/30R21 is 0.02" taller and 50 mm wider; the sidewall is -0.49". At an indicated 60 mph you'd actually be doing 60.0 mph.
Will 325/30R21 fit on 275/40R20 wheels?
No — 275/40R20 needs a 20" wheel and 325/30R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.