+1.90%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 215/45R17, fitting 225/40R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.8 km/h | 25 mph | 25 mph |
| 60 km/h | 61.1 km/h | 37 mph | 38 mph |
| 80 km/h | 81.5 km/h | 50 mph | 51 mph |
| 100 km/h | 101.9 km/h | 62 mph | 63 mph |
| 120 km/h | 122.3 km/h | 75 mph | 76 mph |
Full comparison
215/45R17
Overall diameter625.3 mm · 24.62 in
Section width215 mm · 8.46 in
Sidewall height96.8 mm · 3.81 in
Circumference1964.4 mm · 77.3 in
Revolutions per mile819
Revolutions per km509
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
+1.9%Safe swap — speedometer error is minor
Original
Overall diameter625.3 mm · 24.62 in
Sidewall height96.8 mm
Circumference1,964.4 mm
Revolutions / mile819.2
New
Overall diameter637.2 mm · 25.09 in
Sidewall height90 mm
Circumference2,001.8 mm
Revolutions / mile803.9
Speedometer & odometer error
The new tire changes overall diameter by +1.9%. 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 | 40.8 km/h | 25 mph | 25 mph |
| 60 km/h | 61.1 km/h | 37 mph | 38 mph |
| 80 km/h | 81.5 km/h | 50 mph | 51 mph |
| 100 km/h | 101.9 km/h | 62 mph | 63 mph |
| 120 km/h | 122.3 km/h | 75 mph | 76 mph |
See individual specs: 215/45R17 · 225/40R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.