305/65R18 vs 325/65R18
325/65R18 is outside the ±3% rule as a replacement for 305/65R18: the diameter differs by +3.05%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 325/65R18 on a car calibrated for 305/65R18, the speedometer reads 58.2 mph when you're really doing 60, and the odometer under-counts by 3.0%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.9 mph |
| 45 mph | 46.4 mph |
| 60 mph | 61.8 mph |
| 70 mph | 72.1 mph |
| 80 mph | 82.4 mph |
Other sizes close to 305/65R18
- 305/65R18 vs 275/65R18-4.57%
- 305/65R18 vs 275/70R18-1.35%
- 305/65R18 vs 265/70R18-2.99%
- 305/65R18 vs 255/70R18-4.63%
- 305/65R18 vs 35x12.50R18+4.13%
- 305/65R18 vs 285/65R18-3.05%
- 305/65R18 vs 295/70R18+1.93%
- 305/65R18 vs 33x12.50R18-1.82%
- 305/65R18 vs 285/75R17+0.66%
- 305/65R18 vs 285/75R18+3.63%
- 305/65R18 vs 305/70R18+3.57%
- 305/65R18 vs 285/70R18+0.29%
Questions
Can I replace 305/65R18 with 325/65R18?
325/65R18 is outside the ±3% rule as a replacement for 305/65R18: the diameter differs by +3.05%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 305/65R18 and 325/65R18?
325/65R18 is 1.02" taller and 20 mm wider; the sidewall is +0.51". At an indicated 60 mph you'd actually be doing 61.8 mph.
Will 325/65R18 fit on 305/65R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 325 mm tire and the fender clearance.