Hmmm, something may be wrong with your car; our 2.5 liter manual 2016 Mazda3 is also driven in a 50/50 mix environment of back roads and interstates; the rolling average is about 34 mpg.2.0L manual. Half highway, half back roads for me: 27-28mpg regularly. Makes me miss my Neon.
My wife's CX-5 is also a 2.0L (Skyactiv), also a manual, and gets 32-33mpg with virtually the same mix of driving. That thing is great.
For that matter, my 3.2 liter TL typically averages 26-28 in such mixed driving.