I have a 5.0L. And a Dodge Ram truck. I wouldn't own either if I couldn't afford the gas for them. Gas would have to get over $8 a gallon before I'd reel in my driving habits. Think about it. Just buying a clunker for $5000 to drive around to get better gas mileage won't save you any money unless you stick with it for a decade. You'll have maintenance costs on the clunker, insurance and you'll still be paying for gas even if a little bit less.
Buying a cheaper gas mileage car to replace your DD Mustang just doesn't make sense to me.