[Varn]

Until you get the mechanical right I would not spend the time on putting aero stuff on it. Having a stumble means that you are wasting a lot of gas. Get it to run as well as it can first. Of course in this poor economy nothing wrong with doing totally free stuff like taking your roof rack off. Take out unused seats and other things that you don't need. I think you are on the right track by airing your tires.

If you haven't done it put on new spark plugs and plug wires. You might want to check out the engine compression. Seems like I remember that some of the older GM v8s had trouble with cam wear although I may be thinking older models.

Probably the biggest thing you can do would be to spend the money to get your other car rolling again and save this one for those times you really need it. Also walk or ride a bike more places. (I'm on a roll). Fuel consumption is more than fuel mileage. Just use it less if possible. Keep your speed down to 55. My Econoline gets its best economy at 52-53 mpg according to my ultragauge. The engine gets more efficient above that in top gear but the drag penalty goes up faster. It has a 5.4 v8 automatic. I have more important things than to try and make the Econoline, which is only driven fully loaded. into an economy car. It weighs about 4700 pounds stock empty. Guess I have the luxury of only using it to haul. If I am not hauling my loaded trailer I can get almost 19 mpg for the tank. Running the AC costs about 2 mpg the way I drive.

Good luck to you.