+0.44%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 225/50R17, fitting 225/45R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.2 km/h | 25 mph | 25 mph |
| 60 km/h | 60.3 km/h | 37 mph | 37 mph |
| 80 km/h | 80.4 km/h | 50 mph | 50 mph |
| 100 km/h | 100.4 km/h | 62 mph | 62 mph |
| 120 km/h | 120.5 km/h | 75 mph | 75 mph |
Full comparison
225/50R17
Overall diameter656.8 mm · 25.86 in
Section width225 mm · 8.86 in
Sidewall height112.5 mm · 4.43 in
Circumference2063.4 mm · 81.2 in
Revolutions per mile780
Revolutions per km485
225/45R18
Overall diameter659.7 mm · 25.97 in
Section width225 mm · 8.86 in
Sidewall height101.3 mm · 3.99 in
Circumference2072.5 mm · 81.6 in
Revolutions per mile777
Revolutions per km483
+0.44%Safe swap — speedometer error is minor
Original
Overall diameter656.8 mm · 25.86 in
Sidewall height112.5 mm
Circumference2,063.4 mm
Revolutions / mile779.9
New
Overall diameter659.7 mm · 25.97 in
Sidewall height101.3 mm
Circumference2,072.5 mm
Revolutions / mile776.5
Speedometer & odometer error
The new tire changes overall diameter by +0.44%. 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 | 40.2 km/h | 25 mph | 25 mph |
| 60 km/h | 60.3 km/h | 37 mph | 37 mph |
| 80 km/h | 80.4 km/h | 50 mph | 50 mph |
| 100 km/h | 100.4 km/h | 62 mph | 62 mph |
| 120 km/h | 120.5 km/h | 75 mph | 75 mph |
See individual specs: 225/50R17 · 225/45R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.