Possibly, but not necessarily. It depends whether speedo and mileometer are getting their info through the same route.
I suspect vehicle mfrs have a tendency to make the speedo read high on purpose, as it always reads higher than a SatNav (which works out your speed in real time from your actual movement on the surface of the planet). I can see why they'd do this, as even a slightly "low" reading could be a defence against a speeding charge, leaving them responsible. Don't know whether they do that with the mileage as well.
Your SatNav should also show the mileage travelled, so if you set your trip meter at the start of a programmed journey you can check if your mileometer is also reading high.