-1.83%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 235/55R17, fitting 245/45R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.3 km/h | 25 mph | 24 mph |
| 60 km/h | 58.9 km/h | 37 mph | 37 mph |
| 80 km/h | 78.5 km/h | 50 mph | 49 mph |
| 100 km/h | 98.2 km/h | 62 mph | 61 mph |
| 120 km/h | 117.8 km/h | 75 mph | 73 mph |
Full comparison
235/55R17
Overall diameter690.3 mm · 27.18 in
Section width235 mm · 9.25 in
Sidewall height129.3 mm · 5.09 in
Circumference2168.6 mm · 85.4 in
Revolutions per mile742
Revolutions per km461
245/45R18
Overall diameter677.7 mm · 26.68 in
Section width245 mm · 9.65 in
Sidewall height110.3 mm · 4.34 in
Circumference2129.1 mm · 83.8 in
Revolutions per mile756
Revolutions per km470
-1.83%Safe swap — speedometer error is minor
Original
Overall diameter690.3 mm · 27.18 in
Sidewall height129.3 mm
Circumference2,168.6 mm
Revolutions / mile742.1
New
Overall diameter677.7 mm · 26.68 in
Sidewall height110.3 mm
Circumference2,129.1 mm
Revolutions / mile755.9
Speedometer & odometer error
The new tire changes overall diameter by -1.83%. 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 | 39.3 km/h | 25 mph | 24 mph |
| 60 km/h | 58.9 km/h | 37 mph | 37 mph |
| 80 km/h | 78.5 km/h | 50 mph | 49 mph |
| 100 km/h | 98.2 km/h | 62 mph | 61 mph |
| 120 km/h | 117.8 km/h | 75 mph | 73 mph |
See individual specs: 235/55R17 · 245/45R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.