235/40R18 vs 245/45R18
245/45R18 is outside the ±3% rule as a replacement for 235/40R18: the diameter differs by +5.04%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 245/45R18 on a car calibrated for 235/40R18, the speedometer reads 57.1 mph when you're really doing 60, and the odometer under-counts by 5.0%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.5 mph |
| 45 mph | 47.3 mph |
| 60 mph | 63.0 mph |
| 70 mph | 73.5 mph |
| 80 mph | 84.0 mph |
Other sizes close to 235/40R18
- 235/40R18 vs 245/40R18+1.24%
- 235/40R18 vs 225/45R18+2.25%
- 235/40R18 vs 225/40R18-1.24%
- 235/40R18 vs 235/45R18+3.64%
- 235/40R18 vs 215/50R17+0.25%
- 235/40R18 vs 235/45R17-0.29%
- 235/40R18 vs 255/35R18-1.47%
- 235/40R18 vs 255/40R18+2.48%
- 235/40R18 vs 235/35R19+0.29%
- 235/40R18 vs 275/35R18+0.70%
- 235/40R18 vs 265/35R18-0.39%
- 235/40R18 vs 275/40R18+4.96%
Questions
Can I replace 235/40R18 with 245/45R18?
245/45R18 is outside the ±3% rule as a replacement for 235/40R18: the diameter differs by +5.04%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 235/40R18 and 245/45R18?
245/45R18 is 1.28" taller and 10 mm wider; the sidewall is +0.64". At an indicated 60 mph you'd actually be doing 63.0 mph.
Will 245/45R18 fit on 235/40R18 wheels?
Both use a 18" rim, so the wheel diameter matches; check the wheel's width range for a 245 mm tire and the fender clearance.