285/70R18 vs 305/70R18
305/70R18 is outside the ±3% rule as a replacement for 285/70R18: the diameter differs by +3.27%, 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 285/70R18, the speedometer reads 58.1 mph when you're really doing 60, and the odometer under-counts by 3.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.0 mph |
| 45 mph | 46.5 mph |
| 60 mph | 62.0 mph |
| 70 mph | 72.3 mph |
| 80 mph | 82.6 mph |
Other sizes close to 285/70R18
- 285/70R18 vs 275/65R18-4.85%
- 285/70R18 vs 275/70R18-1.64%
- 285/70R18 vs 265/70R18-3.27%
- 285/70R18 vs 255/70R18-4.91%
- 285/70R18 vs 35x12.50R18+3.83%
- 285/70R18 vs 285/65R18-3.33%
- 285/70R18 vs 295/70R18+1.64%
- 285/70R18 vs 33x12.50R18-2.10%
- 285/70R18 vs 285/75R17+0.36%
- 285/70R18 vs 285/75R18+3.33%
- 285/70R18 vs 325/65R18+2.74%
- 285/70R18 vs 305/65R18-0.29%
Questions
Can I replace 285/70R18 with 305/70R18?
305/70R18 is outside the ±3% rule as a replacement for 285/70R18: the diameter differs by +3.27%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 285/70R18 and 305/70R18?
305/70R18 is 1.10" taller and 20 mm wider; the sidewall is +0.55". At an indicated 60 mph you'd actually be doing 62.0 mph.
Will 305/70R18 fit on 285/70R18 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.