305/60R18 vs 285/70R18
285/70R18 is outside the ±3% rule as a replacement for 305/60R18: the diameter differs by +4.01%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 285/70R18 on a car calibrated for 305/60R18, the speedometer reads 57.7 mph when you're really doing 60, and the odometer under-counts by 4.0%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.2 mph |
| 45 mph | 46.8 mph |
| 60 mph | 62.4 mph |
| 70 mph | 72.8 mph |
| 80 mph | 83.2 mph |
Other sizes close to 305/60R18
- 305/60R18 vs 275/65R18-1.03%
- 305/60R18 vs 275/70R18+2.31%
- 305/60R18 vs 285/70R17+0.92%
- 305/60R18 vs 265/65R18-2.61%
- 305/60R18 vs 265/70R18+0.61%
- 305/60R18 vs 255/70R18-1.09%
- 305/60R18 vs 255/65R18-4.19%
- 305/60R18 vs 285/65R18+0.55%
- 305/60R18 vs 33x12.50R18+1.82%
- 305/60R18 vs 285/60R18-2.92%
- 305/60R18 vs 275/70R17-0.78%
- 305/60R18 vs 305/65R18+3.71%
Questions
Can I replace 305/60R18 with 285/70R18?
285/70R18 is outside the ±3% rule as a replacement for 305/60R18: the diameter differs by +4.01%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 305/60R18 and 285/70R18?
285/70R18 is 1.30" taller and 20 mm narrower; the sidewall is +0.65". At an indicated 60 mph you'd actually be doing 62.4 mph.
Will 285/70R18 fit on 305/60R18 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.