-0.06%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 275/65R18, fitting 255/70R18 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.0 km/h | 25 mph | 25 mph |
| 60 km/h | 60.0 km/h | 37 mph | 37 mph |
| 80 km/h | 80.0 km/h | 50 mph | 50 mph |
| 100 km/h | 99.9 km/h | 62 mph | 62 mph |
| 120 km/h | 119.9 km/h | 75 mph | 75 mph |
Full comparison
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
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
-0.06%Safe swap — speedometer error is minor
Original
Overall diameter814.7 mm · 32.07 in
Sidewall height178.8 mm
Circumference2,559.5 mm
Revolutions / mile628.8
New
Overall diameter814.2 mm · 32.06 in
Sidewall height178.5 mm
Circumference2,557.9 mm
Revolutions / mile629.2
Speedometer & odometer error
The new tire changes overall diameter by -0.06%. 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 km/h | 25 mph | 25 mph |
| 60 km/h | 60 km/h | 37 mph | 37 mph |
| 80 km/h | 80 km/h | 50 mph | 50 mph |
| 100 km/h | 99.9 km/h | 62 mph | 62 mph |
| 120 km/h | 119.9 km/h | 75 mph | 75 mph |
See individual specs: 275/65R18 · 255/70R18. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.