5 speed swap

Play = Pay. Like 5-6K, just for parts. You can buy a bunch of gas for that.

The only real economical options are what I stated earlier, at the expense of labor. For your purposes; can the 5 speed keep the 4. Go slower on the freeway. Keep out of the pedal when your trying to get mileage. Empty the trunk. All these things help.

Plus, It's not like it's your commuter, for that there are plenty of inexpensive cars that get 30-40+ MPG. You can put the difference in the tank of your beast and be happy. Don't let gear lust consume you.