285/30R18 vs 235/40R18
235/40R18 is a safe swap for 285/30R18 by the usual ±3% rule: the diameter differs by +2.71%.
Speedometer and odometer
With 235/40R18 on a car calibrated for 285/30R18, the speedometer reads 58.4 mph when you're really doing 60, and the odometer under-counts by 2.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.8 mph |
| 45 mph | 46.2 mph |
| 60 mph | 61.6 mph |
| 70 mph | 71.9 mph |
| 80 mph | 82.2 mph |
Other sizes close to 285/30R18
- 285/30R18 vs 225/45R17+0.97%
- 285/30R18 vs 245/40R18+3.98%
- 285/30R18 vs 225/40R18+1.43%
- 285/30R18 vs 215/45R17-0.46%
- 285/30R18 vs 255/35R18+1.19%
- 285/30R18 vs 245/40R17-0.06%
- 285/30R18 vs 275/35R18+3.42%
- 285/30R18 vs 265/35R18+2.31%
- 285/30R18 vs 215/45R18+3.58%
- 285/30R18 vs 245/35R18+0.08%
- 285/30R18 vs 215/40R18+0.16%
- 285/30R18 vs 285/35R18+4.54%
Questions
Can I replace 285/30R18 with 235/40R18?
235/40R18 is a safe swap for 285/30R18 by the usual ±3% rule: the diameter differs by +2.71%.
What's the difference between 285/30R18 and 235/40R18?
235/40R18 is 0.67" taller and 50 mm narrower; the sidewall is +0.33". At an indicated 60 mph you'd actually be doing 61.6 mph.
Will 235/40R18 fit on 285/30R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 235 mm tire and the fender clearance.