305/70R18 vs 285/75R18
285/75R18 is a safe swap for 305/70R18 by the usual ±3% rule: the diameter differs by +0.06%.
Speedometer and odometer
With 285/75R18 on a car calibrated for 305/70R18, the speedometer reads 60.0 mph when you're really doing 60, and the odometer under-counts by 0.1%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.0 mph |
| 45 mph | 45.0 mph |
| 60 mph | 60.0 mph |
| 70 mph | 70.0 mph |
| 80 mph | 80.0 mph |
Other sizes close to 305/70R18
Questions
Can I replace 305/70R18 with 285/75R18?
285/75R18 is a safe swap for 305/70R18 by the usual ±3% rule: the diameter differs by +0.06%.
What's the difference between 305/70R18 and 285/75R18?
285/75R18 is 0.02" taller and 20 mm narrower; the sidewall is +0.01". At an indicated 60 mph you'd actually be doing 60.0 mph.
Will 285/75R18 fit on 305/70R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 285 mm tire and the fender clearance.