-2.41%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 255/35R19, fitting 235/40R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.0 km/h | 25 mph | 24 mph |
| 60 km/h | 58.6 km/h | 37 mph | 36 mph |
| 80 km/h | 78.1 km/h | 50 mph | 49 mph |
| 100 km/h | 97.6 km/h | 62 mph | 61 mph |
| 120 km/h | 117.1 km/h | 75 mph | 73 mph |
Full comparison
255/35R19
Overall diameter661.1 mm · 26.03 in
Section width255 mm · 10.04 in
Sidewall height89.3 mm · 3.51 in
Circumference2076.9 mm · 81.8 in
Revolutions per mile775
Revolutions per km481
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
-2.41%Safe swap — speedometer error is minor
Original
Overall diameter661.1 mm · 26.03 in
Sidewall height89.3 mm
Circumference2,076.9 mm
Revolutions / mile774.9
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 -2.41%. 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 km/h | 25 mph | 24 mph |
| 60 km/h | 58.6 km/h | 37 mph | 36 mph |
| 80 km/h | 78.1 km/h | 50 mph | 49 mph |
| 100 km/h | 97.6 km/h | 62 mph | 61 mph |
| 120 km/h | 117.1 km/h | 75 mph | 73 mph |
See individual specs: 255/35R19 · 235/40R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.