325/65R18 vs 305/70R18
305/70R18 is a safe swap for 325/65R18 by the usual ±3% rule: the diameter differs by +0.51%.
Speedometer and odometer
With 305/70R18 on a car calibrated for 325/65R18, the speedometer reads 59.7 mph when you're really doing 60, and the odometer under-counts by 0.5%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.2 mph |
| 60 mph | 60.3 mph |
| 70 mph | 70.4 mph |
| 80 mph | 80.4 mph |
Other sizes close to 325/65R18
Questions
Can I replace 325/65R18 with 305/70R18?
305/70R18 is a safe swap for 325/65R18 by the usual ±3% rule: the diameter differs by +0.51%.
What's the difference between 325/65R18 and 305/70R18?
305/70R18 is 0.18" taller and 20 mm narrower; the sidewall is +0.09". At an indicated 60 mph you'd actually be doing 60.3 mph.
Will 305/70R18 fit on 325/65R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 305 mm tire and the fender clearance.