235/45R18 vs 275/40R18
275/40R18 is a safe swap for 235/45R18 by the usual ±3% rule: the diameter differs by +1.27%.
Speedometer and odometer
With 275/40R18 on a car calibrated for 235/45R18, the speedometer reads 59.2 mph when you're really doing 60, and the odometer under-counts by 1.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.4 mph |
| 45 mph | 45.6 mph |
| 60 mph | 60.8 mph |
| 70 mph | 70.9 mph |
| 80 mph | 81.0 mph |
Other sizes close to 235/45R18
- 235/45R18 vs 225/45R18-1.35%
- 235/45R18 vs 245/40R18-2.32%
- 235/45R18 vs 225/40R18-4.71%
- 235/45R18 vs 245/45R18+1.35%
- 235/45R18 vs 235/50R18+3.51%
- 235/45R18 vs 215/55R17-0.06%
- 235/45R18 vs 235/40R18-3.51%
- 235/45R18 vs 225/50R18+2.02%
- 235/45R18 vs 235/40R19+0.28%
- 235/45R18 vs 255/35R18-4.93%
- 235/45R18 vs 255/40R18-1.12%
- 235/45R18 vs 275/35R19+0.96%
Questions
Can I replace 235/45R18 with 275/40R18?
275/40R18 is a safe swap for 235/45R18 by the usual ±3% rule: the diameter differs by +1.27%.
What's the difference between 235/45R18 and 275/40R18?
275/40R18 is 0.33" taller and 40 mm wider; the sidewall is +0.17". At an indicated 60 mph you'd actually be doing 60.8 mph.
Will 275/40R18 fit on 235/45R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 275 mm tire and the fender clearance.