+2.40%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 235/40R19, fitting 255/45R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 41.0 km/h | 25 mph | 25 mph |
| 60 km/h | 61.4 km/h | 37 mph | 38 mph |
| 80 km/h | 81.9 km/h | 50 mph | 51 mph |
| 100 km/h | 102.4 km/h | 62 mph | 64 mph |
| 120 km/h | 122.9 km/h | 75 mph | 76 mph |
Full comparison
235/40R19
Overall diameter670.6 mm · 26.40 in
Section width235 mm · 9.25 in
Sidewall height94.0 mm · 3.70 in
Circumference2106.8 mm · 82.9 in
Revolutions per mile764
Revolutions per km475
255/45R18
Overall diameter686.7 mm · 27.04 in
Section width255 mm · 10.04 in
Sidewall height114.8 mm · 4.52 in
Circumference2157.3 mm · 84.9 in
Revolutions per mile746
Revolutions per km464
+2.4%Safe swap — speedometer error is minor
Original
Overall diameter670.6 mm · 26.4 in
Sidewall height94 mm
Circumference2,106.8 mm
Revolutions / mile763.9
New
Overall diameter686.7 mm · 27.04 in
Sidewall height114.8 mm
Circumference2,157.3 mm
Revolutions / mile746
Speedometer & odometer error
The new tire changes overall diameter by +2.4%. 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 | 41 km/h | 25 mph | 25 mph |
| 60 km/h | 61.4 km/h | 37 mph | 38 mph |
| 80 km/h | 81.9 km/h | 50 mph | 51 mph |
| 100 km/h | 102.4 km/h | 62 mph | 64 mph |
| 120 km/h | 122.9 km/h | 75 mph | 76 mph |
See individual specs: 235/40R19 · 255/45R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.