I agree with what you typed. BUT...in my car (Lincoln LS), there is no VSS wire, so...I never had the VSS wire hooked up (about two years) with my Avic D3. When going through a couple of tunnels in the Norfolk/Va Beach area, I noticed that the GPS/NAV still tracked the cars forward movement almost dead accurate, and when I came out of the tunnel, the exits were accurately..there.
Go figure.
Some units have a built-in gyroscope/accelerometer (sp?). The D3 probably had one and that would likely explain this phenomenon.