Recently I had an oil change done at a commercial oil change shop. I put in synthetic oil and was really expecting to see at least some mileage differance. After the first fill I was shocked to see my mileage drop. I assumed I had been driving too hard and dismissed it. Next refill, no better!
I then recalled telling the technician that the car had a squirelly air pressure requirement and to leave the tires alone. He said, no worries, I took the pressures off of the gas filler cover.
Turns out my tires were down 4 pounds each. That was enough to knock almost 3 MPG off! It pays to watch the pressure, and to do your own work.