I have a 2015, 5spd. Lots of factors but I hovered around 38-39 mpg during a summer trip last year. Weather was about mid-low 70s. My average speed was between 70-75mph driving back home as well. Tire pressure was 35 psi all around as well. Could be spark plugs. Take a look into the intake too. During the fall, I found 1-2 leaves in my intake O_O...