+0.05%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 235/65R17, fitting 255/55R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.0 km/h | 25 mph | 25 mph |
| 60 km/h | 60.0 km/h | 37 mph | 37 mph |
| 80 km/h | 80.0 km/h | 50 mph | 50 mph |
| 100 km/h | 100.1 km/h | 62 mph | 62 mph |
| 120 km/h | 120.1 km/h | 75 mph | 75 mph |
Full comparison
235/65R17
Overall diameter737.3 mm · 29.03 in
Section width235 mm · 9.25 in
Sidewall height152.8 mm · 6.01 in
Circumference2316.3 mm · 91.2 in
Revolutions per mile695
Revolutions per km432
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
+0.05%Safe swap — speedometer error is minor
Original
Overall diameter737.3 mm · 29.03 in
Sidewall height152.8 mm
Circumference2,316.3 mm
Revolutions / mile694.8
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 +0.05%. 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 km/h | 25 mph | 25 mph |
| 60 km/h | 60 km/h | 37 mph | 37 mph |
| 80 km/h | 80 km/h | 50 mph | 50 mph |
| 100 km/h | 100.1 km/h | 62 mph | 62 mph |
| 120 km/h | 120.1 km/h | 75 mph | 75 mph |
See individual specs: 235/65R17 · 255/55R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.