I am getting 12L/100km (20mpg) for 50/50 highway/city drive, for 100% highway cruising, I get 10L/100km (23.5mpg), I use Sunoco 94.
It's much worse than that in winter.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.