I have a 2002 VW Jetta Wagon TDI, 5spd, as well and have done some tinkering on it....see my car profile for list of items.
I believe the Scan Gauge is great for also showing you what the true water temperature is of the coolant verses the VW stock temp gauge that is ALWAYS wrong. With the Scan Gauge accurate reading and also with adding removeable air blocking on the front grill of the car...you can definitely get better fuel economy and aerodynamics.
The hotter the engine runs...the more efficient fuel is burned. So far I have pushed my temp to 215 deg F (on mnt hill climb) and average around 190 normally. I live in a cold place, especially in winter, but wouldn't want to get above 240 deg F if you can help it (thus the removeable air blocking panels that keep air from going into the radiator/engine area).
Got my Scan Gauge new on eBay for $115 including shipping. Definitely worth it...
Another thing slowing you down is your 16" wheels as they make you higher off the ground (aka worse aerodynamics), more overall tire & rim weight...but this might be an additional 1-2mpg...?
Lastly, thanks for serving our country in Afghanistan and enjoy driving your low mileage VW around (I'm jealous) !
