I think you have taken this thread yet again to an undesirable techinical level.
Cop's don't pace anyone anymore to catch them speeding.And they also don't get there spedometers recalibrated on their vehicles. And did you know that law enforcement,emergency vehiclesand the military use GPS's to monitor their speed. And when they reach a certain speed an email is sent to their supervisors to let them know they have reached that maximum programmed speed. And all with GPS's
But they are not accurate right?
Let me ask you this. What IS more accurate than a GPS????
The OP was simply asking what was more accurate. Speedo or HUD. And through my several tests the HUD has proven to be more accurate.
And that IS all he wanted to know. Not how accurate a GPS actually is.
I don't think I EVER said that a GPS is 100% accurate now have I? And i'm not even sure that if you had an 1/8 of tread ware that the VSS would even be able to register that small of a change.