An old conundrum I've struggled with for ages (even with an early TomTom)!
I'd love to believe the SatNav, because I would guess that the speedo overestimates by a bit (two reasons: makes the car look faster, might also save you points).
Just had to do a speed awareness waste of time (AKA Speed Awareness Course, of course!) for being flashed at 45 in a 40 limit in the middle of Wales, returning from an Irish ferry. It was 02:00am. How P'd O was I? Enormously!!!
They're out to get ya, just play safe!!!