315/70R17 vs 325/65R18
325/65R18 is a safe swap for 315/70R17 by the usual ±3% rule: the diameter differs by +0.79%.
Speedometer and odometer
With 325/65R18 on a car calibrated for 315/70R17, the speedometer reads 59.5 mph when you're really doing 60, and the odometer under-counts by 0.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.4 mph |
| 60 mph | 60.5 mph |
| 70 mph | 70.6 mph |
| 80 mph | 80.6 mph |
Other sizes close to 315/70R17
Questions
Can I replace 315/70R17 with 325/65R18?
325/65R18 is a safe swap for 315/70R17 by the usual ±3% rule: the diameter differs by +0.79%.
What's the difference between 315/70R17 and 325/65R18?
325/65R18 is 0.27" taller and 10 mm wider; the sidewall is -0.36". At an indicated 60 mph you'd actually be doing 60.5 mph.
Will 325/65R18 fit on 315/70R17 wheels?
No — 315/70R17 needs a 17" wheel and 325/65R18 a 18" wheel. This is a plus/minus-size swap that changes wheels too.