+1.60%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 245/60R18, fitting 255/60R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.6 km/h | 25 mph | 25 mph |
| 60 km/h | 61.0 km/h | 37 mph | 38 mph |
| 80 km/h | 81.3 km/h | 50 mph | 51 mph |
| 100 km/h | 101.6 km/h | 62 mph | 63 mph |
| 120 km/h | 121.9 km/h | 75 mph | 76 mph |
Full comparison
245/60R18
Overall diameter751.2 mm · 29.57 in
Section width245 mm · 9.65 in
Sidewall height147.0 mm · 5.79 in
Circumference2360.0 mm · 92.9 in
Revolutions per mile682
Revolutions per km424
255/60R18
Overall diameter763.2 mm · 30.05 in
Section width255 mm · 10.04 in
Sidewall height153.0 mm · 6.02 in
Circumference2397.7 mm · 94.4 in
Revolutions per mile671
Revolutions per km417
+1.6%Safe swap — speedometer error is minor
Original
Overall diameter751.2 mm · 29.57 in
Sidewall height147 mm
Circumference2,360 mm
Revolutions / mile681.9
New
Overall diameter763.2 mm · 30.05 in
Sidewall height153 mm
Circumference2,397.7 mm
Revolutions / mile671.2
Speedometer & odometer error
The new tire changes overall diameter by +1.6%. 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 | 40.6 km/h | 25 mph | 25 mph |
| 60 km/h | 61 km/h | 37 mph | 38 mph |
| 80 km/h | 81.3 km/h | 50 mph | 51 mph |
| 100 km/h | 101.6 km/h | 62 mph | 63 mph |
| 120 km/h | 121.9 km/h | 75 mph | 76 mph |
See individual specs: 245/60R18 · 255/60R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.