-0.25%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 215/50R17, fitting 235/40R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.9 km/h | 25 mph | 25 mph |
| 60 km/h | 59.9 km/h | 37 mph | 37 mph |
| 80 km/h | 79.8 km/h | 50 mph | 50 mph |
| 100 km/h | 99.8 km/h | 62 mph | 62 mph |
| 120 km/h | 119.7 km/h | 75 mph | 74 mph |
Full comparison
215/50R17
Overall diameter646.8 mm · 25.46 in
Section width215 mm · 8.46 in
Sidewall height107.5 mm · 4.23 in
Circumference2032.0 mm · 80.0 in
Revolutions per mile792
Revolutions per km492
235/40R18
Overall diameter645.2 mm · 25.40 in
Section width235 mm · 9.25 in
Sidewall height94.0 mm · 3.70 in
Circumference2027.0 mm · 79.8 in
Revolutions per mile794
Revolutions per km493
-0.25%Safe swap — speedometer error is minor
Original
Overall diameter646.8 mm · 25.46 in
Sidewall height107.5 mm
Circumference2,032 mm
Revolutions / mile792
New
Overall diameter645.2 mm · 25.4 in
Sidewall height94 mm
Circumference2,027 mm
Revolutions / mile794
Speedometer & odometer error
The new tire changes overall diameter by -0.25%. 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 | 39.9 km/h | 25 mph | 25 mph |
| 60 km/h | 59.9 km/h | 37 mph | 37 mph |
| 80 km/h | 79.8 km/h | 50 mph | 50 mph |
| 100 km/h | 99.8 km/h | 62 mph | 62 mph |
| 120 km/h | 119.7 km/h | 75 mph | 74 mph |
See individual specs: 215/50R17 · 235/40R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.