+1.13%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 245/45R17, fitting 225/45R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.5 km/h | 25 mph | 25 mph |
| 60 km/h | 60.7 km/h | 37 mph | 38 mph |
| 80 km/h | 80.9 km/h | 50 mph | 50 mph |
| 100 km/h | 101.1 km/h | 62 mph | 63 mph |
| 120 km/h | 121.4 km/h | 75 mph | 75 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
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
+1.13%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 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 +1.13%. 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 | 40.5 km/h | 25 mph | 25 mph |
| 60 km/h | 60.7 km/h | 37 mph | 38 mph |
| 80 km/h | 80.9 km/h | 50 mph | 50 mph |
| 100 km/h | 101.1 km/h | 62 mph | 63 mph |
| 120 km/h | 121.4 km/h | 75 mph | 75 mph |
See individual specs: 245/45R17 · 225/45R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.