Maybe I'm naive (so my wife says) but I bet if you changed your air filter and plugs and did some general tune up kinda things, you would see an improvement.
Funny to read a posting about a thirsty vitara. I always go to the car show up here in Toronto, and you know, the GV is always listed as the most fuel efficient vehicles in the country. Maybe it was the most fuel efficient 4x4 or something, but it was certainly up there, although Im sure that hybrid Prius takes the crown now for most overall fuel efficient vehicle.
I think that the real problem is not the engines efficiency, but the fact that the body is essentially a box...not aerodynamic at all. Therefore my solution is o bolt on one of these
or one of these
and the incredible reduction in your coefficient of drag should just about outweigh all that extra fiberglass you just bolted on. Good Luck!
!