There are few things that is good to know:

Tolerance for car instruments is 10% (max), so at 100 km/h it is about 10 km/h.
This is not error only from instrument, tires can be new or old, pressure can be higher or lower, and so on..., which influence the result.

To avoid error in such way that someone is driving faster then allowed (when instrument is showing that speed), all instruments are set to show about +10% (max).

Result is that ALL car instruments are ALWAYS showing speed somewhere between actual speed and +10%

So when navigation is showing LOWER speed than car instrument, it is likely that actual speed is shown by navigation.