-11.71%Not recommended — recalibrate the speedometer
Speedometer error
With a speedometer calibrated to 325/65R20, fitting 285/55R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 35.3 km/h | 25 mph | 22 mph |
| 60 km/h | 53.0 km/h | 37 mph | 33 mph |
| 80 km/h | 70.6 km/h | 50 mph | 44 mph |
| 100 km/h | 88.3 km/h | 62 mph | 55 mph |
| 120 km/h | 105.9 km/h | 75 mph | 66 mph |
Full comparison
325/65R20
Overall diameter930.5 mm · 36.63 in
Section width325 mm · 12.80 in
Sidewall height211.3 mm · 8.32 in
Circumference2923.3 mm · 115.1 in
Revolutions per mile551
Revolutions per km342
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
-11.71%Not recommended — recalibrate the speedometer
Original
Overall diameter930.5 mm · 36.63 in
Sidewall height211.3 mm
Circumference2,923.3 mm
Revolutions / mile550.5
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 -11.71%. 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 | 35.3 km/h | 25 mph | 22 mph |
| 60 km/h | 53 km/h | 37 mph | 33 mph |
| 80 km/h | 70.6 km/h | 50 mph | 44 mph |
| 100 km/h | 88.3 km/h | 62 mph | 55 mph |
| 120 km/h | 105.9 km/h | 75 mph | 66 mph |
See individual specs: 325/65R20 · 285/55R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.