As you have calculated that the mpg is lower than before, after a long trip abroad. My initial thought would be that France use a different fuel mix.
Do you know how it compares to UK? There could very well be a different percentage of biofuel or ethanol in the mix.
I usually buy fuel in Norway as it has better milage, compared to Sweden. That goes for petrol as well as diesel.
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Johannes
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Audi S8 -99, Pearlescent, Alcantara/silk napa leather, Burr walnut insert, Alcantara roof lining upper pack
A8 32V engine
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Jeep Grand Cherokee -98
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