The SGII is pricey but it is very powerful. With it, I have jumped from 38mpg to my current tank of 43 (best trip was 46 mpg) because I can surgically tailor my driving to get better FE.
By far, the best and cheapest mod you can do is to change you -- the driver. I jumped from 33 to 38mpg: reading the 105 tips, hanging out here and at
cleanmpg, listening to NPR and just being more relaxed with the road. I pumped my tires up to 40psi and coast a lot more and cleaned up some of the aerodynamics by covering/removing any glaring "air catches". I'll work on an upper grille block next.
I don't usually look at signatures because I look at the left margin first (then I cry at the kickass FE people are getting that I wish I was... but I'll be there someday!!! I believe in the Scangauge)