-9.73%Not recommended — recalibrate the speedometer
Speedometer error
With a speedometer calibrated to 285/65R20, fitting 285/50R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 36.1 km/h | 25 mph | 22 mph |
| 60 km/h | 54.2 km/h | 37 mph | 34 mph |
| 80 km/h | 72.2 km/h | 50 mph | 45 mph |
| 100 km/h | 90.3 km/h | 62 mph | 56 mph |
| 120 km/h | 108.3 km/h | 75 mph | 67 mph |
Full comparison
285/65R20
Overall diameter878.5 mm · 34.59 in
Section width285 mm · 11.22 in
Sidewall height185.3 mm · 7.29 in
Circumference2759.9 mm · 108.7 in
Revolutions per mile583
Revolutions per km362
285/50R20
Overall diameter793.0 mm · 31.22 in
Section width285 mm · 11.22 in
Sidewall height142.5 mm · 5.61 in
Circumference2491.3 mm · 98.1 in
Revolutions per mile646
Revolutions per km401
-9.73%Not recommended — recalibrate the speedometer
Original
Overall diameter878.5 mm · 34.59 in
Sidewall height185.3 mm
Circumference2,759.9 mm
Revolutions / mile583.1
New
Overall diameter793 mm · 31.22 in
Sidewall height142.5 mm
Circumference2,491.3 mm
Revolutions / mile646
Speedometer & odometer error
The new tire changes overall diameter by -9.73%. 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 | 36.1 km/h | 25 mph | 22 mph |
| 60 km/h | 54.2 km/h | 37 mph | 34 mph |
| 80 km/h | 72.2 km/h | 50 mph | 45 mph |
| 100 km/h | 90.3 km/h | 62 mph | 56 mph |
| 120 km/h | 108.3 km/h | 75 mph | 67 mph |
See individual specs: 285/65R20 · 285/50R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.