+1.37%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 285/70R17, fitting 275/70R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.5 km/h | 25 mph | 25 mph |
| 60 km/h | 60.8 km/h | 37 mph | 38 mph |
| 80 km/h | 81.1 km/h | 50 mph | 50 mph |
| 100 km/h | 101.4 km/h | 62 mph | 63 mph |
| 120 km/h | 121.6 km/h | 75 mph | 76 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
275/70R18
Overall diameter842.2 mm · 33.16 in
Section width275 mm · 10.83 in
Sidewall height192.5 mm · 7.58 in
Circumference2645.8 mm · 104.2 in
Revolutions per mile608
Revolutions per km378
+1.37%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 diameter842.2 mm · 33.16 in
Sidewall height192.5 mm
Circumference2,645.8 mm
Revolutions / mile608.3
Speedometer & odometer error
The new tire changes overall diameter by +1.37%. 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 | 40.5 km/h | 25 mph | 25 mph |
| 60 km/h | 60.8 km/h | 37 mph | 38 mph |
| 80 km/h | 81.1 km/h | 50 mph | 50 mph |
| 100 km/h | 101.4 km/h | 62 mph | 63 mph |
| 120 km/h | 121.6 km/h | 75 mph | 76 mph |
See individual specs: 285/70R17 · 275/70R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.