These are computer indicated figures, which do seem over by about 1mpg or so.
Still very impressive.
Driving my 306 HDi (book figure 55mpg, but reality 44mpg standard, and 40mpg tuned up), on the same route at much the same speeds saw not much more mpg.
Throw in the fuel cost difference, and the BMW Z4 is actually cheaper all round if driven steadily.
Diesels really don't make sense once they have enough power to be entertaining.