I did a touch of research.
It looks like that car is rated at 26 mpg freeway (24 per 2008 EPA). If you're only getting 18 mpg . . . I'd wonder if something isn't right.
1984 (California) LINCOLN-MERCURY GRAND MARQUIS - MPG and Detailed Vehicle Information
Just something to keep in mind is that if you get really ambitious and the car is emissions exempt, and you have the Megasquirt add-on to control the ignition as well, you could do your own lean-burn engine.
http://en.wikipedia.org/wiki/Lean_burn
Finally, I did some paint doodles that might give you some ideas for aero-mods.