[Big Dave]

I think a 40 MPG Crown Vic can be done, but baby you are gonna have to throw the kitchen sink ($$$$) at it.

Gabi has a good start and slamming a vehicle has worked for me. Look to the basjoos-mobile. To get to 40 MPG you will have to be as relentless as basjoos was. Do everything: belly pan, air dam, grille block, radical boat-tail. Maybe some radical narrowing of the aft part of the cabin.

To get to 40 MPG, you will have to be equally ruthless about weight. Lose the spare/jack. Modern tires are far more durable than back in the 60s. I've been driving since LBJ was president and have never had anything but slow leaks. It is far more likely your tires will fail from dry rot. Do like airplanes. Replace the glass windows with plexiglas. Pitch the sound system overboard. Drag racers have ten thousand tricks. Plug into the racing community.

Narrower (say 215 or 195 width) LRR (maybe Goodyear Fuel Max Assurance or Michelin Hydroedge) will help. Be sure to smooth out the wheels.

Gotta lose that gas-pig engine. Gotta lose the slushbox. I have often thought a Cummins 4BT3.9 or M-B OW617 mated to a T-56 would be a very hot setup for MPG. Get the numerically lowest rear axle gears you can find.

40 MPG from a Crown Vic is ENTIRELY DO-ABLE, but don't kid yourself it will be easy or cheap.