+1.71%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 255/55R18, fitting 245/65R17 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.7 km/h | 25 mph | 25 mph |
| 60 km/h | 61.0 km/h | 37 mph | 38 mph |
| 80 km/h | 81.4 km/h | 50 mph | 51 mph |
| 100 km/h | 101.7 km/h | 62 mph | 63 mph |
| 120 km/h | 122.0 km/h | 75 mph | 76 mph |
Full comparison
255/55R18
Overall diameter737.7 mm · 29.04 in
Section width255 mm · 10.04 in
Sidewall height140.3 mm · 5.52 in
Circumference2317.6 mm · 91.2 in
Revolutions per mile694
Revolutions per km431
245/65R17
Overall diameter750.3 mm · 29.54 in
Section width245 mm · 9.65 in
Sidewall height159.3 mm · 6.27 in
Circumference2357.1 mm · 92.8 in
Revolutions per mile683
Revolutions per km424
+1.71%Safe swap — speedometer error is minor
Original
Overall diameter737.7 mm · 29.04 in
Sidewall height140.3 mm
Circumference2,317.6 mm
Revolutions / mile694.4
New
Overall diameter750.3 mm · 29.54 in
Sidewall height159.3 mm
Circumference2,357.1 mm
Revolutions / mile682.8
Speedometer & odometer error
The new tire changes overall diameter by +1.71%. 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 | 40.7 km/h | 25 mph | 25 mph |
| 60 km/h | 61 km/h | 37 mph | 38 mph |
| 80 km/h | 81.4 km/h | 50 mph | 51 mph |
| 100 km/h | 101.7 km/h | 62 mph | 63 mph |
| 120 km/h | 122 km/h | 75 mph | 76 mph |
See individual specs: 255/55R18 · 245/65R17. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.