+0.56%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 265/30R19, fitting 235/40R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.2 km/h | 25 mph | 25 mph |
| 60 km/h | 60.3 km/h | 37 mph | 37 mph |
| 80 km/h | 80.4 km/h | 50 mph | 50 mph |
| 100 km/h | 100.6 km/h | 62 mph | 62 mph |
| 120 km/h | 120.7 km/h | 75 mph | 75 mph |
Full comparison
265/30R19
Overall diameter641.6 mm · 25.26 in
Section width265 mm · 10.43 in
Sidewall height79.5 mm · 3.13 in
Circumference2015.6 mm · 79.4 in
Revolutions per mile798
Revolutions per km496
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
+0.56%Safe swap — speedometer error is minor
Original
Overall diameter641.6 mm · 25.26 in
Sidewall height79.5 mm
Circumference2,015.6 mm
Revolutions / mile798.4
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 +0.56%. 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 | 40.2 km/h | 25 mph | 25 mph |
| 60 km/h | 60.3 km/h | 37 mph | 37 mph |
| 80 km/h | 80.4 km/h | 50 mph | 50 mph |
| 100 km/h | 100.6 km/h | 62 mph | 62 mph |
| 120 km/h | 120.7 km/h | 75 mph | 75 mph |
See individual specs: 265/30R19 · 235/40R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.