305/40R20 vs 315/35R21
315/35R21 is a safe swap for 305/40R20 by the usual ±3% rule: the diameter differs by +0.25%.
Speedometer and odometer
With 315/35R21 on a car calibrated for 305/40R20, the speedometer reads 59.8 mph when you're really doing 60, and the odometer under-counts by 0.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.1 mph |
| 60 mph | 60.2 mph |
| 70 mph | 70.2 mph |
| 80 mph | 80.2 mph |
Other sizes close to 305/40R20
- 305/40R20 vs 245/45R20-3.12%
- 305/40R20 vs 255/45R20-1.93%
- 305/40R20 vs 255/50R20+1.46%
- 305/40R20 vs 275/40R20-3.19%
- 305/40R20 vs 265/50R20+2.79%
- 305/40R20 vs 245/50R20+0.13%
- 305/40R20 vs 275/45R20+0.47%
- 305/40R20 vs 255/55R20+4.85%
- 305/40R20 vs 235/55R20+1.93%
- 305/40R20 vs 245/55R19+0.01%
- 305/40R20 vs 265/45R20-0.73%
- 305/40R20 vs 265/50R19-0.59%
Questions
Can I replace 305/40R20 with 315/35R21?
315/35R21 is a safe swap for 305/40R20 by the usual ±3% rule: the diameter differs by +0.25%.
What's the difference between 305/40R20 and 315/35R21?
315/35R21 is 0.07" taller and 10 mm wider; the sidewall is -0.46". At an indicated 60 mph you'd actually be doing 60.2 mph.
Will 315/35R21 fit on 305/40R20 wheels?
No — 305/40R20 needs a 20" wheel and 315/35R21 a 21" wheel. This is a plus/minus-size swap that changes wheels too.