+0.06%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 205/50R17, fitting 225/40R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.0 km/h | 25 mph | 25 mph |
| 60 km/h | 60.0 km/h | 37 mph | 37 mph |
| 80 km/h | 80.1 km/h | 50 mph | 50 mph |
| 100 km/h | 100.1 km/h | 62 mph | 62 mph |
| 120 km/h | 120.1 km/h | 75 mph | 75 mph |
Full comparison
205/50R17
Overall diameter636.8 mm · 25.07 in
Section width205 mm · 8.07 in
Sidewall height102.5 mm · 4.04 in
Circumference2000.6 mm · 78.8 in
Revolutions per mile804
Revolutions per km500
225/40R18
Overall diameter637.2 mm · 25.09 in
Section width225 mm · 8.86 in
Sidewall height90.0 mm · 3.54 in
Circumference2001.8 mm · 78.8 in
Revolutions per mile804
Revolutions per km500
+0.06%Safe swap — speedometer error is minor
Original
Overall diameter636.8 mm · 25.07 in
Sidewall height102.5 mm
Circumference2,000.6 mm
Revolutions / mile804.4
New
Overall diameter637.2 mm · 25.09 in
Sidewall height90 mm
Circumference2,001.8 mm
Revolutions / mile803.9
Speedometer & odometer error
The new tire changes overall diameter by +0.06%. 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 km/h | 25 mph | 25 mph |
| 60 km/h | 60 km/h | 37 mph | 37 mph |
| 80 km/h | 80.1 km/h | 50 mph | 50 mph |
| 100 km/h | 100.1 km/h | 62 mph | 62 mph |
| 120 km/h | 120.1 km/h | 75 mph | 75 mph |
See individual specs: 205/50R17 · 225/40R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.