You may also find a light weight car won't pulse and glide very well, so you'll be stuck cruising in gear. There has been the odd guy on here who's ballasted their car for improved P&G. Although I wouldn't go to that extreme, I do much prefer hypermiling a car that gives me a bit of momentum to use to my advantage.
I dropped off 800lbs of materials yesterday, and got (~10%) better economy loaded than I did on the return trip. I was cruising at 80% load at 2800rpm (right about BSFC) so using my fuel very efficiently
Not claiming that as a scientific test, but it goes to show how much weight you can remove and not really see a benefit.