It may not be a bad idea to wait a little longer, but I'm not convinced there isn't something going on. I took a look at my spreadsheet - for the first 2K miles I had my Durango (2014, V-6, AWD) my MPG numbers were 21.4, 21.3, 19.0, 20.3, and 20.1. The first couple were mostly highway miles, but the others were around town. They replaced the engine last October (Lifetime MaxCare for the win!) and the first 2K gave me MPG numbers of 17.3, 18.0, 20.5, 23.2, 22.5, 18.6, with those three middle ones mostly highway miles. Even for a new engine, his sound very low.