Story: I recently bought a 2013 Civic and during my first oil change I noticed that the person before me had completely removed the big splash guard that goes underneath the engine (a big plastic sheet-like piece that runs the width of the car and from the front fender to about 1/3rd of the way to the rear; it has 2 metal sheets attached to it).
I eventually tracked one down at a junk yard and saw an immediate difference of 1-2 MPG in every kind of driving condition. My city-only trios went from 26 to 28, mixed went from 36 to 37 or 38 (it varied). And once a month I take a long drive upstate where I just set the cruise to 65 the whole time; I used to max out at 47 MPG, now I'm maxing out at 48.
Anyways, I know sometimes people lose these splash guards for whatever reason, but make sure you keep yours on, as it's well worth having.