What to believe (ScanGauge & GPS vs. speedometer)

[reformed]

As far as the portables, it varies, usually Delphi, Harmon Kardon, or Garmin lately. Sometimes Pioneers version the avic S1. The in-dash units are mostly Pioneer, some Eclipse, some Alpine (years ago) and a few Panasonic. I personally switched from the Delphi, to the Pioneer in-dash, and its a world of difference.

I also forgot to mention that the in-dash units have a built in gyro sensor to compliment the GPS, which makes them more accurate as far as location goes. You seem to know more about the actual workings of GPS than I do, I just put them in, take them for a test drive to make sure everything functions within tolerance, and send them on their way. So I will say I believe you that speed and location are different.