285/35R21 vs 285/40R20
285/40R20 is a safe swap for 285/35R21 by the usual ±3% rule: the diameter differs by +0.42%.
Speedometer and odometer
With 285/40R20 on a car calibrated for 285/35R21, the speedometer reads 59.7 mph when you're really doing 60, and the odometer under-counts by 0.4%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.2 mph |
| 60 mph | 60.3 mph |
| 70 mph | 70.3 mph |
| 80 mph | 80.3 mph |
Other sizes close to 285/35R21
- 285/35R21 vs 245/45R20-0.60%
- 285/35R21 vs 255/45R20+0.63%
- 285/35R21 vs 275/40R20-0.67%
- 285/35R21 vs 265/40R21+1.71%
- 285/35R21 vs 315/35R20-0.60%
- 285/35R21 vs 275/35R21-0.96%
- 285/35R21 vs 295/35R21+0.96%
- 285/35R21 vs 255/40R21+0.61%
- 285/35R21 vs 265/35R21-1.91%
- 285/35R21 vs 245/35R21-3.82%
- 285/35R21 vs 295/40R21+4.98%
- 285/35R21 vs 285/40R21+3.89%
Questions
Can I replace 285/35R21 with 285/40R20?
285/40R20 is a safe swap for 285/35R21 by the usual ±3% rule: the diameter differs by +0.42%.
What's the difference between 285/35R21 and 285/40R20?
285/40R20 is 0.12" taller and 0 mm wider; the sidewall is +0.56". At an indicated 60 mph you'd actually be doing 60.3 mph.
Will 285/40R20 fit on 285/35R21 wheels?
No — 285/35R21 needs a 21" wheel and 285/40R20 a 20" wheel. This is a plus/minus-size swap that changes wheels too.