+0.96%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 225/40R18, fitting 235/45R17 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.4 km/h | 25 mph | 25 mph |
| 60 km/h | 60.6 km/h | 37 mph | 38 mph |
| 80 km/h | 80.8 km/h | 50 mph | 50 mph |
| 100 km/h | 101.0 km/h | 62 mph | 63 mph |
| 120 km/h | 121.1 km/h | 75 mph | 75 mph |
Full comparison
225/40R18
Overall diameter637.2 mm · 25.09 in
Section width225 mm · 8.86 in
Sidewall height90.0 mm · 3.54 in
Circumference2001.8 mm · 78.8 in
Revolutions per mile804
Revolutions per km500
235/45R17
Overall diameter643.3 mm · 25.33 in
Section width235 mm · 9.25 in
Sidewall height105.8 mm · 4.16 in
Circumference2021.0 mm · 79.6 in
Revolutions per mile796
Revolutions per km495
+0.96%Safe swap — speedometer error is minor
Original
Overall diameter637.2 mm · 25.09 in
Sidewall height90 mm
Circumference2,001.8 mm
Revolutions / mile803.9
New
Overall diameter643.3 mm · 25.33 in
Sidewall height105.8 mm
Circumference2,021 mm
Revolutions / mile796.3
Speedometer & odometer error
The new tire changes overall diameter by +0.96%. 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 | 40.4 km/h | 25 mph | 25 mph |
| 60 km/h | 60.6 km/h | 37 mph | 38 mph |
| 80 km/h | 80.8 km/h | 50 mph | 50 mph |
| 100 km/h | 101 km/h | 62 mph | 63 mph |
| 120 km/h | 121.1 km/h | 75 mph | 75 mph |
See individual specs: 225/40R18 · 235/45R17. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.