325/60R18 vs 305/70R18
305/70R18 is outside the ±3% rule as a replacement for 325/60R18: the diameter differs by +4.37%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 305/70R18 on a car calibrated for 325/60R18, the speedometer reads 57.5 mph when you're really doing 60, and the odometer under-counts by 4.4%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.3 mph |
| 45 mph | 47.0 mph |
| 60 mph | 62.6 mph |
| 70 mph | 73.1 mph |
| 80 mph | 83.5 mph |
Other sizes close to 325/60R18
- 325/60R18 vs 275/65R18-3.84%
- 325/60R18 vs 275/70R18-0.59%
- 325/60R18 vs 265/70R18-2.24%
- 325/60R18 vs 255/70R18-3.90%
- 325/60R18 vs 35x12.50R18+4.93%
- 325/60R18 vs 285/65R18-2.30%
- 325/60R18 vs 295/70R18+2.71%
- 325/60R18 vs 295/70R17-0.28%
- 325/60R18 vs 33x12.50R18-1.06%
- 325/60R18 vs 285/75R18+4.43%
- 325/60R18 vs 325/65R18+3.84%
- 325/60R18 vs 255/80R17-0.87%
Questions
Can I replace 325/60R18 with 305/70R18?
305/70R18 is outside the ±3% rule as a replacement for 325/60R18: the diameter differs by +4.37%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 325/60R18 and 305/70R18?
305/70R18 is 1.46" taller and 20 mm narrower; the sidewall is +0.73". At an indicated 60 mph you'd actually be doing 62.6 mph.
Will 305/70R18 fit on 325/60R18 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.