How many miles after break in did you get that good MPG?
My 2014 V6 manual Mustang only has 1400 miles for now. I usually get only 16 MPG with half highway and half city driving. I do some heavy acceleration occasionally.
Your engine is still WAY too tight. Don't even look at MPGs until it's broken in around 5,000 miles or so. Drive it hard. Don't baby a new engine!