The new tire changes overall diameter by +3.64%. With a speedometer calibrated to the original tire, that same percentage is how far the speedometer and odometer read off — a taller tire means your true speed is higher than indicated and the odometer under-counts distance.
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 41.5 km/h | 25 mph | 26 mph |
| 60 km/h | 62.2 km/h | 37 mph | 39 mph |
| 80 km/h | 82.9 km/h | 50 mph | 52 mph |
| 100 km/h | 103.6 km/h | 62 mph | 64 mph |
| 120 km/h | 124.4 km/h | 75 mph | 77 mph |
Commonly compared with 235/40R18
- 235/40R18 vs 215/50R17 · 215/50R17 specs
- 235/40R18 vs 235/35R19 · 235/35R19 specs
- 235/40R18 vs 235/45R17 · 235/45R17 specs
- 235/40R18 vs 265/30R19 · 265/30R19 specs
- 235/40R18 vs 245/45R17 · 245/45R17 specs
- 235/40R18 vs 225/40R18 · 225/40R18 specs
- 235/40R18 vs 245/40R18 · 245/40R18 specs
- 235/40R18 vs 205/50R17 · 205/50R17 specs
FAQ
What is the overall diameter of a 235/40R18 tire?
A 235/40R18 tire has an overall diameter of about 25.40 inches (645 mm) and a section width of 235 mm.
What sizes can replace a 235/40R18 tire?
Sizes within about ±3% of 235/40R18's 25.4-inch diameter are the closest matches, such as 215/50R17, 235/35R19, 235/45R17. Use the calculator above to check the exact speedometer effect of any substitute.
Fitment & trouble codes
Changing tire size by more than about 3% shifts the speedometer, odometer and, on some vehicles, ABS/traction calibration — which can set wheel-speed-sensor or vehicle-speed codes. See C1234 (wheel speed sensor) and C0050 (rear wheel speed) if a warning light appears after a size change.