+1.20%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 235/55R18, fitting 225/65R17 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 | 81.0 km/h | 50 mph | 50 mph |
| 100 km/h | 101.2 km/h | 62 mph | 63 mph |
| 120 km/h | 121.4 km/h | 75 mph | 75 mph |
Full comparison
235/55R18
Overall diameter715.7 mm · 28.18 in
Section width235 mm · 9.25 in
Sidewall height129.3 mm · 5.09 in
Circumference2248.4 mm · 88.5 in
Revolutions per mile716
Revolutions per km445
225/65R17
Overall diameter724.3 mm · 28.52 in
Section width225 mm · 8.86 in
Sidewall height146.3 mm · 5.76 in
Circumference2275.5 mm · 89.6 in
Revolutions per mile707
Revolutions per km439
+1.2%Safe swap — speedometer error is minor
Original
Overall diameter715.7 mm · 28.18 in
Sidewall height129.3 mm
Circumference2,248.4 mm
Revolutions / mile715.8
New
Overall diameter724.3 mm · 28.52 in
Sidewall height146.3 mm
Circumference2,275.5 mm
Revolutions / mile707.3
Speedometer & odometer error
The new tire changes overall diameter by +1.2%. 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 | 81 km/h | 50 mph | 50 mph |
| 100 km/h | 101.2 km/h | 62 mph | 63 mph |
| 120 km/h | 121.4 km/h | 75 mph | 75 mph |
See individual specs: 235/55R18 · 225/65R17. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.