EP:
  Is your TRIP COMPUTER part of the car, or an add-on?  I mentioned this the other day, thinking that people would probably behave differenly if the MPG info was right in their face all the time.

  Is there a device that connects to the engine's computer-data-plug that can yield that readout? *(most US cars have that now, for servicers to do diagnostics, right?)

Bob Fiske

It came with the car (part of the VW dash cluster).  I can't say if it's standard equipment or not, but it probably should be.
http://www.scangauge.com/

http://www.jcwhitney.com/   Search for "Miles"

MILES-PER-GALLON GAUGE
JC Whitney Price: $27.99

The JC Whitney gauge is a vaccuum gauge in miles-per-gallon gauge clothing.  Could probably help in keeping you from romping on the gas pedal (probably just wig out if you have a turbo), but I doubt it'll really tell you what you're getting for your gallon.