If you are doing a steady speed on a straight road, the sat-nav should be spot on. However they don't adjust quickly enough to give an accurate instaneous reading during changes in speed and/or direction. Tall buildings don't help as the sat-nav can miss some satellite bleeps and then it just estimates your position until it gets another reading.
So they are perfect for calibrating your speedo on a long straight (eg a motorway), but not, in my view, suitable as an everyday speedo.
I can't say I speak with any real authority on the subject, but this how I believe it works.
Fjemog
