I think that the Speedo sensing with a per vehicle map of volume response would be a practical implementation of the intent I had.
There could be a graph displayed where the X axis has incremental speeds (say every 16.09kph ) each increment would have a delta volume. The owner sets those according to his percived needs. Another way, maybe better, would be to associate a specific eq with each speed.

Either way the software interpolates between fixed speed increments.

I must confess I don't have a clue how the road speed could be collected from the total variaty of cars on the road today.

The gingerbread is: An empeg visiual of a road endlessly appearing from over the hill with road signs showing track info flashing by at X miles and hour.

-- Glenn