-0.20%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 235/60R18, fitting 255/55R18 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 | 119.8 km/h | 75 mph | 74 mph |
Full comparison
235/60R18
Overall diameter739.2 mm · 29.10 in
Section width235 mm · 9.25 in
Sidewall height141.0 mm · 5.55 in
Circumference2322.3 mm · 91.4 in
Revolutions per mile693
Revolutions per km431
255/55R18
Overall diameter737.7 mm · 29.04 in
Section width255 mm · 10.04 in
Sidewall height140.3 mm · 5.52 in
Circumference2317.6 mm · 91.2 in
Revolutions per mile694
Revolutions per km431
-0.2%Safe swap — speedometer error is minor
Original
Overall diameter739.2 mm · 29.1 in
Sidewall height141 mm
Circumference2,322.3 mm
Revolutions / mile693
New
Overall diameter737.7 mm · 29.04 in
Sidewall height140.3 mm
Circumference2,317.6 mm
Revolutions / mile694.4
Speedometer & odometer error
The new tire changes overall diameter by -0.2%. 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 | 119.8 km/h | 75 mph | 74 mph |
See individual specs: 235/60R18 · 255/55R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.