+0.13%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 245/45R19, fitting 245/40R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 40.1 km/h | 25 mph | 25 mph |
| 60 km/h | 60.1 km/h | 37 mph | 37 mph |
| 80 km/h | 80.1 km/h | 50 mph | 50 mph |
| 100 km/h | 100.1 km/h | 62 mph | 62 mph |
| 120 km/h | 120.2 km/h | 75 mph | 75 mph |
Full comparison
245/45R19
Overall diameter703.1 mm · 27.68 in
Section width245 mm · 9.65 in
Sidewall height110.3 mm · 4.34 in
Circumference2208.9 mm · 87.0 in
Revolutions per mile729
Revolutions per km453
245/40R20
Overall diameter704.0 mm · 27.72 in
Section width245 mm · 9.65 in
Sidewall height98.0 mm · 3.86 in
Circumference2211.7 mm · 87.1 in
Revolutions per mile728
Revolutions per km452
+0.13%Safe swap — speedometer error is minor
Original
Overall diameter703.1 mm · 27.68 in
Sidewall height110.3 mm
Circumference2,208.9 mm
Revolutions / mile728.6
New
Overall diameter704 mm · 27.72 in
Sidewall height98 mm
Circumference2,211.7 mm
Revolutions / mile727.7
Speedometer & odometer error
The new tire changes overall diameter by +0.13%. 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 | 40.1 km/h | 25 mph | 25 mph |
| 60 km/h | 60.1 km/h | 37 mph | 37 mph |
| 80 km/h | 80.1 km/h | 50 mph | 50 mph |
| 100 km/h | 100.1 km/h | 62 mph | 62 mph |
| 120 km/h | 120.2 km/h | 75 mph | 75 mph |
See individual specs: 245/45R19 · 245/40R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.