2007 Mustang GT (S197)
I average 60 Miles per day (48 highway), 365.25 days a year. After an inadvertent aero-mod (ripped off the rear bumper cover on I-95... frak'n snow) my average fuel economy went from 24.6 MPG to 26 something.
Over the past seven days, I've gotten a little frisky after work: I gutted the interior of 180 lbs., fully ducted the radiator, blocked the lower grille, and fab'ed a diffuser that reaches back to the "K" member.
Here, you can see the bottom of the duct work for the radiator:
I did this mid-tank and now my latest hand-calculated average was 27.8 mpg. I’m curious to see what the next tank is going to be.