AU Fuel Econ

[99_falcon]

I drive a AU falcon (I6) and the L/100 on the highway is said to be around 6.8L/100Km. I know this is an average under ideal driving conditions (also a well maintained and tuned car etc.). Now my car has been maintained and looked after etc. Now I've calculated that the 550km round trip to Twizel and back should theoretically use 37.4L, give or take a couple of L but when I last drove to Twizel and back. The figures I got were more of that of the city driving average (11.5L/100Km), around 63L. Odd how much the difference was. Am I missing something here?