+1.85%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 225/65R17, fitting 255/55R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.7 km/h | 25 mph | 25 mph |
| 60 km/h | 61.1 km/h | 37 mph | 38 mph |
| 80 km/h | 81.5 km/h | 50 mph | 51 mph |
| 100 km/h | 101.9 km/h | 62 mph | 63 mph |
| 120 km/h | 122.2 km/h | 75 mph | 76 mph |
Full comparison
225/65R17
Overall diameter724.3 mm · 28.52 in
Section width225 mm · 8.86 in
Sidewall height146.3 mm · 5.76 in
Circumference2275.5 mm · 89.6 in
Revolutions per mile707
Revolutions per km439
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
+1.85%Safe swap — speedometer error is minor
Original
Overall diameter724.3 mm · 28.52 in
Sidewall height146.3 mm
Circumference2,275.5 mm
Revolutions / mile707.3
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 +1.85%. 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 | 40.7 km/h | 25 mph | 25 mph |
| 60 km/h | 61.1 km/h | 37 mph | 38 mph |
| 80 km/h | 81.5 km/h | 50 mph | 51 mph |
| 100 km/h | 101.9 km/h | 62 mph | 63 mph |
| 120 km/h | 122.2 km/h | 75 mph | 76 mph |
See individual specs: 225/65R17 · 255/55R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.