-1.20%Safe swap — speedometer error is minor
Speedometer error
With a speedometer calibrated to 245/50R20, fitting 295/40R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 39.5 km/h | 25 mph | 25 mph |
| 60 km/h | 59.3 km/h | 37 mph | 37 mph |
| 80 km/h | 79.0 km/h | 50 mph | 49 mph |
| 100 km/h | 98.8 km/h | 62 mph | 61 mph |
| 120 km/h | 118.6 km/h | 75 mph | 74 mph |
Full comparison
245/50R20
Overall diameter753.0 mm · 29.65 in
Section width245 mm · 9.65 in
Sidewall height122.5 mm · 4.82 in
Circumference2365.6 mm · 93.1 in
Revolutions per mile680
Revolutions per km423
295/40R20
Overall diameter744.0 mm · 29.29 in
Section width295 mm · 11.61 in
Sidewall height118.0 mm · 4.65 in
Circumference2337.3 mm · 92.0 in
Revolutions per mile689
Revolutions per km428
-1.2%Safe swap — speedometer error is minor
Original
Overall diameter753 mm · 29.65 in
Sidewall height122.5 mm
Circumference2,365.6 mm
Revolutions / mile680.3
New
Overall diameter744 mm · 29.29 in
Sidewall height118 mm
Circumference2,337.3 mm
Revolutions / mile688.5
Speedometer & odometer error
The new tire changes overall diameter by -1.2%. 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 | 39.5 km/h | 25 mph | 25 mph |
| 60 km/h | 59.3 km/h | 37 mph | 37 mph |
| 80 km/h | 79 km/h | 50 mph | 49 mph |
| 100 km/h | 98.8 km/h | 62 mph | 61 mph |
| 120 km/h | 118.6 km/h | 75 mph | 74 mph |
See individual specs: 245/50R20 · 295/40R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.