I was trying to do the same thing with my boxy boat at one time. Just overall driving like an old man got me 23-25mpg. I had the computer read 28mpg at one time, but rush hour killed that quick.
Do you know what kind of ratio gears are in the rear? If you're cruising on the highway mostly, 2.73s or even 2.55s will do nicely with a 4 speed auto.
I had skirts on my Marq, but that was before I was worrying about gas mileage and they got ripped off after an accidental off-road adventure.
