+3.07%Borderline — noticeable speedometer/odometer error
Speedometer error
With a speedometer calibrated to 235/55R18, fitting 255/55R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 41.2 km/h | 25 mph | 26 mph |
| 60 km/h | 61.8 km/h | 37 mph | 38 mph |
| 80 km/h | 82.5 km/h | 50 mph | 51 mph |
| 100 km/h | 103.1 km/h | 62 mph | 64 mph |
| 120 km/h | 123.7 km/h | 75 mph | 77 mph |
Full comparison
235/55R18
Overall diameter715.7 mm · 28.18 in
Section width235 mm · 9.25 in
Sidewall height129.3 mm · 5.09 in
Circumference2248.4 mm · 88.5 in
Revolutions per mile716
Revolutions per km445
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
+3.07%Borderline — noticeable speedometer/odometer error
Original
Overall diameter715.7 mm · 28.18 in
Sidewall height129.3 mm
Circumference2,248.4 mm
Revolutions / mile715.8
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 +3.07%. 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 | 41.2 km/h | 25 mph | 26 mph |
| 60 km/h | 61.8 km/h | 37 mph | 38 mph |
| 80 km/h | 82.5 km/h | 50 mph | 51 mph |
| 100 km/h | 103.1 km/h | 62 mph | 64 mph |
| 120 km/h | 123.7 km/h | 75 mph | 77 mph |
See individual specs: 235/55R18 · 255/55R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.