-1.94%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 285/70R17, fitting 275/65R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.2 km/h | 25 mph | 24 mph |
| 60 km/h | 58.8 km/h | 37 mph | 37 mph |
| 80 km/h | 78.4 km/h | 50 mph | 49 mph |
| 100 km/h | 98.1 km/h | 62 mph | 61 mph |
| 120 km/h | 117.7 km/h | 75 mph | 73 mph |
Full comparison
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
275/65R18
Overall diameter814.7 mm · 32.07 in
Section width275 mm · 10.83 in
Sidewall height178.8 mm · 7.04 in
Circumference2559.5 mm · 100.8 in
Revolutions per mile629
Revolutions per km391
-1.94%Safe swap — speedometer error is minor
Original
Overall diameter830.8 mm · 32.71 in
Sidewall height199.5 mm
Circumference2,610 mm
Revolutions / mile616.6
New
Overall diameter814.7 mm · 32.07 in
Sidewall height178.8 mm
Circumference2,559.5 mm
Revolutions / mile628.8
Speedometer & odometer error
The new tire changes overall diameter by -1.94%. 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 | 39.2 km/h | 25 mph | 24 mph |
| 60 km/h | 58.8 km/h | 37 mph | 37 mph |
| 80 km/h | 78.4 km/h | 50 mph | 49 mph |
| 100 km/h | 98.1 km/h | 62 mph | 61 mph |
| 120 km/h | 117.7 km/h | 75 mph | 73 mph |
See individual specs: 285/70R17 · 275/65R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.