-0.14%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 245/40R18, fitting 245/45R17 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.9 km/h | 25 mph | 25 mph |
| 60 km/h | 59.9 km/h | 37 mph | 37 mph |
| 80 km/h | 79.9 km/h | 50 mph | 50 mph |
| 100 km/h | 99.9 km/h | 62 mph | 62 mph |
| 120 km/h | 119.8 km/h | 75 mph | 74 mph |
Full comparison
245/40R18
Overall diameter653.2 mm · 25.72 in
Section width245 mm · 9.65 in
Sidewall height98.0 mm · 3.86 in
Circumference2052.1 mm · 80.8 in
Revolutions per mile784
Revolutions per km487
245/45R17
Overall diameter652.3 mm · 25.68 in
Section width245 mm · 9.65 in
Sidewall height110.3 mm · 4.34 in
Circumference2049.3 mm · 80.7 in
Revolutions per mile785
Revolutions per km488
-0.14%Safe swap — speedometer error is minor
Original
Overall diameter653.2 mm · 25.72 in
Sidewall height98 mm
Circumference2,052.1 mm
Revolutions / mile784.2
New
Overall diameter652.3 mm · 25.68 in
Sidewall height110.3 mm
Circumference2,049.3 mm
Revolutions / mile785.3
Speedometer & odometer error
The new tire changes overall diameter by -0.14%. 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 | 39.9 km/h | 25 mph | 25 mph |
| 60 km/h | 59.9 km/h | 37 mph | 37 mph |
| 80 km/h | 79.9 km/h | 50 mph | 50 mph |
| 100 km/h | 99.9 km/h | 62 mph | 62 mph |
| 120 km/h | 119.8 km/h | 75 mph | 74 mph |
See individual specs: 245/40R18 · 245/45R17. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.