-2.26%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 215/55R17, fitting 245/40R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.1 km/h | 25 mph | 24 mph |
| 60 km/h | 58.6 km/h | 37 mph | 36 mph |
| 80 km/h | 78.2 km/h | 50 mph | 49 mph |
| 100 km/h | 97.7 km/h | 62 mph | 61 mph |
| 120 km/h | 117.3 km/h | 75 mph | 73 mph |
Full comparison
215/55R17
Overall diameter668.3 mm · 26.31 in
Section width215 mm · 8.46 in
Sidewall height118.3 mm · 4.66 in
Circumference2099.5 mm · 82.7 in
Revolutions per mile767
Revolutions per km476
245/40R18
Overall diameter653.2 mm · 25.72 in
Section width245 mm · 9.65 in
Sidewall height98.0 mm · 3.86 in
Circumference2052.1 mm · 80.8 in
Revolutions per mile784
Revolutions per km487
-2.26%Safe swap — speedometer error is minor
Original
Overall diameter668.3 mm · 26.31 in
Sidewall height118.3 mm
Circumference2,099.5 mm
Revolutions / mile766.5
New
Overall diameter653.2 mm · 25.72 in
Sidewall height98 mm
Circumference2,052.1 mm
Revolutions / mile784.2
Speedometer & odometer error
The new tire changes overall diameter by -2.26%. 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 | 39.1 km/h | 25 mph | 24 mph |
| 60 km/h | 58.6 km/h | 37 mph | 36 mph |
| 80 km/h | 78.2 km/h | 50 mph | 49 mph |
| 100 km/h | 97.7 km/h | 62 mph | 61 mph |
| 120 km/h | 117.3 km/h | 75 mph | 73 mph |
See individual specs: 215/55R17 · 245/40R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.