Adagio you are correct in saying diesels can be very economical and are in general much better than a petrol Euro. The world record is held by a Peugeot 307 that got something like 3.5l/100km!!! Driving around Australia.
I'm hardly challenging any records here. As I said my consumption was achieved driving in a straight line on a flat smooth road at a constant speed.
Vehicles like the 307 are able to do it in a variety of conditions, mainly thanks to the torque at low revs they have. They deal with hills a lot better than the Euro when crusing at 100km/hr.
Either way, I'm tempted to plan a run of my own (not quite round Australia) perhaps Brisbane to Sydney or something.




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