Not calculated, however I took the Jeep on a 150ish mile trip this past weekend, resetting the overhead computer as I merged onto the highway. I haven't reset my average for several months, and it had settled in the 15.2-15.4 range in my mix of driving. After my trip, I was showing an average of 18 flat, which consisted of full A/C use both ways, some heavy idle time in traffic, and 75ish MPH cruise speeds. I was also facing 20mph head winds on my way home.
Not scientific, but not bad for 245k.