I drive with a heavy foot and typically get 13.5 mixed with 80% city and 20% highway. When I do take long highway trips I have seen as high as 22.1, but that was a rare 1 time event (doing 80mph down hill with a tailwind). If I keep it right around 65-70 on freeway driving, I can see 18-19 all day long and net about 500 miles on a tank of fuel, but once you crest the 70mph mark you start dropping like a rock on the mileage (at least mine does).
I hope this helps, don't be scared by the numbers as most of us on here do not buy these for the mileage anyway. I got mine for one reason, she goes like a ***** ape... and the sound is amazing as well.
Cheers,
Joe