+5.92%Not recommended — recalibrate the speedometer
Speedometer error
With a speedometer calibrated to 285/65R20, fitting 325/65R20 makes it read as follows:
| Speedo shows | True speed | Speedo shows | True speed |
|---|---|---|---|
| 40 km/h | 42.4 km/h | 25 mph | 26 mph |
| 60 km/h | 63.6 km/h | 37 mph | 39 mph |
| 80 km/h | 84.7 km/h | 50 mph | 53 mph |
| 100 km/h | 105.9 km/h | 62 mph | 66 mph |
| 120 km/h | 127.1 km/h | 75 mph | 79 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
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
+5.92%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 diameter930.5 mm · 36.63 in
Sidewall height211.3 mm
Circumference2,923.3 mm
Revolutions / mile550.5
Speedometer & odometer error
The new tire changes overall diameter by +5.92%. 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 | 42.4 km/h | 25 mph | 26 mph |
| 60 km/h | 63.6 km/h | 37 mph | 39 mph |
| 80 km/h | 84.7 km/h | 50 mph | 53 mph |
| 100 km/h | 105.9 km/h | 62 mph | 66 mph |
| 120 km/h | 127.1 km/h | 75 mph | 79 mph |
See individual specs: 285/65R20 · 325/65R20. A change this size can affect ABS/speed calibration — see wheel-speed codes if a light appears.