-1.50%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 245/70R17, fitting 255/60R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.4 km/h | 25 mph | 24 mph |
| 60 km/h | 59.1 km/h | 37 mph | 37 mph |
| 80 km/h | 78.8 km/h | 50 mph | 49 mph |
| 100 km/h | 98.5 km/h | 62 mph | 61 mph |
| 120 km/h | 118.2 km/h | 75 mph | 73 mph |
Full comparison
245/70R17
Overall diameter774.8 mm · 30.50 in
Section width245 mm · 9.65 in
Sidewall height171.5 mm · 6.75 in
Circumference2434.1 mm · 95.8 in
Revolutions per mile661
Revolutions per km411
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
-1.5%Safe swap — speedometer error is minor
Original
Overall diameter774.8 mm · 30.5 in
Sidewall height171.5 mm
Circumference2,434.1 mm
Revolutions / mile661.2
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 -1.5%. 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 | 39.4 km/h | 25 mph | 24 mph |
| 60 km/h | 59.1 km/h | 37 mph | 37 mph |
| 80 km/h | 78.8 km/h | 50 mph | 49 mph |
| 100 km/h | 98.5 km/h | 62 mph | 61 mph |
| 120 km/h | 118.2 km/h | 75 mph | 73 mph |
See individual specs: 245/70R17 · 255/60R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.