The fuel economy of my 2004 Holden Rodeo V6 has been around 16-20 L/100 km. I reckon that is atrocious, but Holden people told me it was normal, that they were thirsty. In frustration I had my and my mechanic mate put his diagnostic scanner on it, and lo and behold, it showed dodgy oxygen sensors. After installing new sensors it is a new vehicle, running smoother, and using between 10-14 L/100 km. I bet there are a lot of vehicles out there with the same problem?
It's odd that Holden's diagnostic equipment failed to find the problem oxygen sensors. In any case it is good to get an outcome, and it's something any other owner with a fuel consumption problem should think about checking.