-1.84%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 255/70R18, fitting 285/60R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.3 km/h | 25 mph | 24 mph |
| 60 km/h | 58.9 km/h | 37 mph | 37 mph |
| 80 km/h | 78.5 km/h | 50 mph | 49 mph |
| 100 km/h | 98.2 km/h | 62 mph | 61 mph |
| 120 km/h | 117.8 km/h | 75 mph | 73 mph |
Full comparison
255/70R18
Overall diameter814.2 mm · 32.06 in
Section width255 mm · 10.04 in
Sidewall height178.5 mm · 7.03 in
Circumference2557.9 mm · 100.7 in
Revolutions per mile629
Revolutions per km391
285/60R18
Overall diameter799.2 mm · 31.46 in
Section width285 mm · 11.22 in
Sidewall height171.0 mm · 6.73 in
Circumference2510.8 mm · 98.8 in
Revolutions per mile641
Revolutions per km398
-1.84%Safe swap — speedometer error is minor
Original
Overall diameter814.2 mm · 32.06 in
Sidewall height178.5 mm
Circumference2,557.9 mm
Revolutions / mile629.2
New
Overall diameter799.2 mm · 31.46 in
Sidewall height171 mm
Circumference2,510.8 mm
Revolutions / mile641
Speedometer & odometer error
The new tire changes overall diameter by -1.84%. 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 | 39.3 km/h | 25 mph | 24 mph |
| 60 km/h | 58.9 km/h | 37 mph | 37 mph |
| 80 km/h | 78.5 km/h | 50 mph | 49 mph |
| 100 km/h | 98.2 km/h | 62 mph | 61 mph |
| 120 km/h | 117.8 km/h | 75 mph | 73 mph |
See individual specs: 255/70R18 · 285/60R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.