-1.56%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 225/55R17, fitting 235/45R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.4 km/h | 25 mph | 24 mph |
| 60 km/h | 59.1 km/h | 37 mph | 37 mph |
| 80 km/h | 78.8 km/h | 50 mph | 49 mph |
| 100 km/h | 98.4 km/h | 62 mph | 61 mph |
| 120 km/h | 118.1 km/h | 75 mph | 73 mph |
Full comparison
225/55R17
Overall diameter679.3 mm · 26.74 in
Section width225 mm · 8.86 in
Sidewall height123.8 mm · 4.87 in
Circumference2134.1 mm · 84.0 in
Revolutions per mile754
Revolutions per km469
235/45R18
Overall diameter668.7 mm · 26.33 in
Section width235 mm · 9.25 in
Sidewall height105.8 mm · 4.16 in
Circumference2100.8 mm · 82.7 in
Revolutions per mile766
Revolutions per km476
-1.56%Safe swap — speedometer error is minor
Original
Overall diameter679.3 mm · 26.74 in
Sidewall height123.8 mm
Circumference2,134.1 mm
Revolutions / mile754.1
New
Overall diameter668.7 mm · 26.33 in
Sidewall height105.8 mm
Circumference2,100.8 mm
Revolutions / mile766.1
Speedometer & odometer error
The new tire changes overall diameter by -1.56%. 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 | 39.4 km/h | 25 mph | 24 mph |
| 60 km/h | 59.1 km/h | 37 mph | 37 mph |
| 80 km/h | 78.8 km/h | 50 mph | 49 mph |
| 100 km/h | 98.4 km/h | 62 mph | 61 mph |
| 120 km/h | 118.1 km/h | 75 mph | 73 mph |
See individual specs: 225/55R17 · 235/45R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.