Thanks for the confirmation.Yes, there's 1 mile difference between my GPS and Formentor. It's almost as good as Audi, which had 0.6 mile (reading taken from OBD reader app) difference. In other cars, it was usually 2 miles at 70mph, Mercedes had 10 km difference at 250km/h.
Interestingly, American cars are spot on speed, comparing to navigation. They must have different legal requirements.
There goes my "buffer" I took into account when going slightly above the speed limit.