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.

Check the tire pressure monthly as the outside temperatures decrease. Every 10 degree drop will decrease your tire pressure by 2lbs.

Klaus
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(V♂LV♂s 1995 854T, 1998 V70R)

Some people have complained that the max pressures listed on the gasoline door are so high, the car feels "bouncy", but there is no question that high pressure helps the tires lose less energy to deformation and frictional heating.