-6.49%Not recommended — recalibrate the speedometer
Speedometer error
With a speedometer calibrated to 285/65R20, fitting 285/55R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 37.4 km/h | 25 mph | 23 mph |
| 60 km/h | 56.1 km/h | 37 mph | 35 mph |
| 80 km/h | 74.8 km/h | 50 mph | 46 mph |
| 100 km/h | 93.5 km/h | 62 mph | 58 mph |
| 120 km/h | 112.2 km/h | 75 mph | 70 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
285/55R20
Overall diameter821.5 mm · 32.34 in
Section width285 mm · 11.22 in
Sidewall height156.8 mm · 6.17 in
Circumference2580.8 mm · 101.6 in
Revolutions per mile624
Revolutions per km387
-6.49%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 diameter821.5 mm · 32.34 in
Sidewall height156.8 mm
Circumference2,580.8 mm
Revolutions / mile623.6
Speedometer & odometer error
The new tire changes overall diameter by -6.49%. 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 | 37.4 km/h | 25 mph | 23 mph |
| 60 km/h | 56.1 km/h | 37 mph | 35 mph |
| 80 km/h | 74.8 km/h | 50 mph | 46 mph |
| 100 km/h | 93.5 km/h | 62 mph | 58 mph |
| 120 km/h | 112.2 km/h | 75 mph | 70 mph |
See individual specs: 285/65R20 · 285/55R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.