-0.55%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 225/50R17, fitting 245/40R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.8 km/h | 25 mph | 25 mph |
| 60 km/h | 59.7 km/h | 37 mph | 37 mph |
| 80 km/h | 79.6 km/h | 50 mph | 49 mph |
| 100 km/h | 99.5 km/h | 62 mph | 62 mph |
| 120 km/h | 119.3 km/h | 75 mph | 74 mph |
Full comparison
225/50R17
Overall diameter656.8 mm · 25.86 in
Section width225 mm · 8.86 in
Sidewall height112.5 mm · 4.43 in
Circumference2063.4 mm · 81.2 in
Revolutions per mile780
Revolutions per km485
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
-0.55%Safe swap — speedometer error is minor
Original
Overall diameter656.8 mm · 25.86 in
Sidewall height112.5 mm
Circumference2,063.4 mm
Revolutions / mile779.9
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 -0.55%. 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 | 39.8 km/h | 25 mph | 25 mph |
| 60 km/h | 59.7 km/h | 37 mph | 37 mph |
| 80 km/h | 79.6 km/h | 50 mph | 49 mph |
| 100 km/h | 99.5 km/h | 62 mph | 62 mph |
| 120 km/h | 119.3 km/h | 75 mph | 74 mph |
See individual specs: 225/50R17 · 245/40R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.