I agree with Shorttimer. Go for the aero mods. I can't remember what the Rabbit value was in the way of Coefficient of Friction but it seemed pretty good for an 80s design. But the air dam up front, the underpan, a rear wheel well cover and something to break up the vacuum at the tail is where you might find the gains you need to consistently get the over 50 mpg you want.
We already have the gears, the compression and the means to control footage pressure working for us.
Q Why Carry All of That Diesel Around???
A Because running out sucks balls
I did a search for CD on the Rabbit & came up with .42 Not so red hot. Ended up over at Ecomodder for several hours & stumbled onto a few really good threads & some really good tips by the Grand Master of them all 'Basjoos'. This is one area which I have planed to do a lot of work, and don't worry, if I'm successful, you all will be hearing about it and hopefully joining in.