My 4 month average is 22.1 mpg using Premium. Not bad, but I'm out in the boonies so mostly hwy driving. I just leave it in Normal mode. Eco is too laggy, and Sport is too twitchy on the gas pedal (based on feedback from the boss in the passenger seat).
A lot better than the 5.7 Hemi in my Ram that I couldn't get an avg over 15 if my life depended on it.
2015 K900 - Black on Black, VIP package