+2.04%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 255/70R18, fitting 285/70R17 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.8 km/h | 25 mph | 25 mph |
| 60 km/h | 61.2 km/h | 37 mph | 38 mph |
| 80 km/h | 81.6 km/h | 50 mph | 51 mph |
| 100 km/h | 102.0 km/h | 62 mph | 63 mph |
| 120 km/h | 122.4 km/h | 75 mph | 76 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
285/70R17
Overall diameter830.8 mm · 32.71 in
Section width285 mm · 11.22 in
Sidewall height199.5 mm · 7.85 in
Circumference2610.0 mm · 102.8 in
Revolutions per mile617
Revolutions per km383
+2.04%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 diameter830.8 mm · 32.71 in
Sidewall height199.5 mm
Circumference2,610 mm
Revolutions / mile616.6
Speedometer & odometer error
The new tire changes overall diameter by +2.04%. 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 | 40.8 km/h | 25 mph | 25 mph |
| 60 km/h | 61.2 km/h | 37 mph | 38 mph |
| 80 km/h | 81.6 km/h | 50 mph | 51 mph |
| 100 km/h | 102 km/h | 62 mph | 63 mph |
| 120 km/h | 122.4 km/h | 75 mph | 76 mph |
See individual specs: 255/70R18 · 285/70R17. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.