+0.80%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 255/45R18, fitting 235/50R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.3 km/h | 25 mph | 25 mph |
| 60 km/h | 60.5 km/h | 37 mph | 38 mph |
| 80 km/h | 80.6 km/h | 50 mph | 50 mph |
| 100 km/h | 100.8 km/h | 62 mph | 63 mph |
| 120 km/h | 121.0 km/h | 75 mph | 75 mph |
Full comparison
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
235/50R18
Overall diameter692.2 mm · 27.25 in
Section width235 mm · 9.25 in
Sidewall height117.5 mm · 4.63 in
Circumference2174.6 mm · 85.6 in
Revolutions per mile740
Revolutions per km460
+0.8%Safe swap — speedometer error is minor
Original
Overall diameter686.7 mm · 27.04 in
Sidewall height114.8 mm
Circumference2,157.3 mm
Revolutions / mile746
New
Overall diameter692.2 mm · 27.25 in
Sidewall height117.5 mm
Circumference2,174.6 mm
Revolutions / mile740.1
Speedometer & odometer error
The new tire changes overall diameter by +0.8%. 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 | 40.3 km/h | 25 mph | 25 mph |
| 60 km/h | 60.5 km/h | 37 mph | 38 mph |
| 80 km/h | 80.6 km/h | 50 mph | 50 mph |
| 100 km/h | 100.8 km/h | 62 mph | 63 mph |
| 120 km/h | 121 km/h | 75 mph | 75 mph |
See individual specs: 255/45R18 · 235/50R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.