My Digifant VW inline four (ABA from a 92/93 Jetta) again has an operative
Oxygen sensor working with the Digifant engine management system.   With
the Oxygen sensor unplugged (due to wiring harness issues, etc) I lost ~ 3
miles per gallon of fuel economy.   The engine still ran strong and had no
drive-ability issues without a working O2 sensor, but it does use more
gasoline.   I went from 22 to 24mpg, I am using Jim's Fuelly App now to
track my gas.  Very helpful data.

   The "Fuelly" app  is not a real pain to use.  There is even a secondary
Android app that allows me to input my gas stops right from the pump using
my smart phone..It sends the information over to the main app on my
laptop.  You can get your trends in fuel consumption, your cost per mile is
kept track of and any changes you make can be traced, like I just did while
running with no o2 sensor for a few tanks of gas.

   I think my own fairly good fuel economy driving my 5sp 2.0 liter ABA
inline van, I think it's enhanced by my style of driving.   I try to keep
my highway cruising speed to around 60-65mph and I try to maintain a
constant speed with no sudden accelerations (Yeah, I know, Sudden
Acceleration and Vanagon don't belong in the same sentence, but...)   I try
to anticipate long grades and move my speed and rpms up as I approach, so
that I get onto the climb in the "meat" of my power band...or around
3500rpms in 5th gear...that allows me to pretty much hold speed at 70, or
if I encounter traffic or wind, I'll drop back to 4th gear and go up at
60mph at around 4000rpms.

  A 'smooth' driving style does much to improve your gas mileage...When you
monitor your results using one of these apps that are availbale free
online, you will be surprised at how you can really affect what goes out of
your wallet and into the tank of your vehicles.