-1.09%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 245/45R17, fitting 235/40R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.6 km/h | 25 mph | 25 mph |
| 60 km/h | 59.3 km/h | 37 mph | 37 mph |
| 80 km/h | 79.1 km/h | 50 mph | 49 mph |
| 100 km/h | 98.9 km/h | 62 mph | 61 mph |
| 120 km/h | 118.7 km/h | 75 mph | 74 mph |
Full comparison
245/45R17
Overall diameter652.3 mm · 25.68 in
Section width245 mm · 9.65 in
Sidewall height110.3 mm · 4.34 in
Circumference2049.3 mm · 80.7 in
Revolutions per mile785
Revolutions per km488
235/40R18
Overall diameter645.2 mm · 25.40 in
Section width235 mm · 9.25 in
Sidewall height94.0 mm · 3.70 in
Circumference2027.0 mm · 79.8 in
Revolutions per mile794
Revolutions per km493
-1.09%Safe swap — speedometer error is minor
Original
Overall diameter652.3 mm · 25.68 in
Sidewall height110.3 mm
Circumference2,049.3 mm
Revolutions / mile785.3
New
Overall diameter645.2 mm · 25.4 in
Sidewall height94 mm
Circumference2,027 mm
Revolutions / mile794
Speedometer & odometer error
The new tire changes overall diameter by -1.09%. 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 | 39.6 km/h | 25 mph | 25 mph |
| 60 km/h | 59.3 km/h | 37 mph | 37 mph |
| 80 km/h | 79.1 km/h | 50 mph | 49 mph |
| 100 km/h | 98.9 km/h | 62 mph | 61 mph |
| 120 km/h | 118.7 km/h | 75 mph | 74 mph |
See individual specs: 245/45R17 · 235/40R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.