Not too much. My average, according to MDrive is 13.5 L/100km. That was a 800 kph drive, averaging ~120-130kph. A "few" hard accelerations, and a couple of steady drives @ 130-140 kph. I tried to vary the speed/rpm as much as I could, but it was hard to do with a 800 km highway drive home.
All in all.......I don't care what the fuel economy, or the apparent lack there of ! , is!
Grey Matter:2008 E92 M3:608/425. SG/PS
Dark Matter:2015 F80 M3:495/505. AB/CB