I know this is an older thread, but I was out taking some pics, and I found a perfect road to do an "ideal" test of the 540's fuel consumption, so here's a quick video
90km/h with Cruise control on, 6th gear, perfectly flat road, 21 degrees C.
Reset the OBC when I started (as shown)
And yes that reads 44MPG (imperial) or 6.4L/100km
That's hybrid territory baby.