I tought the the VSS was going to be used for increasing/decreasing the volume of the unit as the speed goes up or down like my current Ford unit is doing.

Went trough the user guide of the X920BT and found no indication about this feature, is it available or not?

Are other models functional with speed volume ?

What is the VSS used for if not for this ?

The VSS tracks your position on the GPS even if you completely lose sat signal (i.e. parking garage or tunnel). This is of course after it calibrates which is taking forever on my Z110bt.

That means that no AVIC radios have the automatic volume function?

Yes.

YES meaning none of them have automatic volume, or YES some have ?

Not clear on this one.....

None have that feature.

I have the X910BT, and it uses the VSS wire to adjust the navigation audio. Or maybe it gets the speed from the satellites and gets louder as I go faster. Unfortunately, the radio and other sources remain at a constant level.

The F and previous X series will do speed sensitive navi voice prompts but that is it. The X920BT does not include any speed sensitive volume for any feature.