So far (only 2021 miles as of yesterday) its been regularly between 17mpg and 18mpg, but that is because I drive almost all winding back roads to and from my office.
When I've taken it exclusively on a couple I-5 freeway trips its gotten about 21mpg.
I'm trying a couple different brand of gas, out here my Chev Tahoe gets 1 to 2mpg better with Union 76 Unleaded+ then it does with Chevron Unleaded+. May seem strange to you all but I tested it on my Tahoe when gas cost over $4/gallon on several controlled trips up and down I-5 within days of each other (had to go to and from Portland-Seattle 3 times in a 10 day period and got the same results each time). I mention it because I've been mainly using Chevron Unleaded+ in my Challenger (the station is closer to my route to work) so I don't know if it would have the same positive effects if I used Union 76 Unleaded+ or Super.