Little doubt the UK tank size is 80 litres and it seems many JDMs may also be the same size although suggestions that some are down to 60 litres (BONSAI spec sheets).
For Dblock to get 560-600 miles to a tank, using say 75 litres, that is around 36 miles to the gallon. I have no doubt that if I drove around as slowly as possible I could get that figure as I have just seen 31.5 mpg on the 600 odd mile drive back from Scotland - mainly motorway and averaging 58mph bearing in mind some hold-ups on the way.
But the ZED is such a wonderful drive I cannot see how anyone can fully enjoy its capabilities and average over 25 mpg - it just does not add up.
But hey, if thats the pleasure you get from a ZED then good luck to you but I do feel you are really missing what driving a ZED is all about