AU1 fuel consumption

[titan 4380]

I have a 1998 AU1 Fairmont with the 4.0l 6 cylinder. I am the second owner and I know for a fact that the car has been very well serviced all of its life and has spent most of its 300,000 km life on highway cycle. Since owning it I have driven it like an old fart and drive rurally at no faster than 80 kmph and 100 when I am on the highway. I run the car on 98RON fuel, but cannot seem to get any better economy than 11.3 L/100km. This seems to be absurdly high for a car that has been treated like a baby all of its life. Even with the ECON mode selected, the car does not show an improvement in economy. The car starts and runs like a Swiss watch except for in cold weather where the car will start ok, but then stall even though a second start will fix the problem. Any clues why I am getting such rotten fuel economy?