I've got a German car with a 4-cylinder diesel engine and a 5-speed.  It climbs mountains and accelerates to freeway speeds from 40 MPH in top gear.  Its top speed is at least 100 MPH.  Slow down?  It sure won't make you.

The only thing that helps is the real-time MPG reading on the trip computer.

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.