Well, before we go any further, what's your goal? Lowest overall fuel consumption, or to save $? The answer's going to be different depending on your goal.
If it's strictly a financial decision, you need to crunch the numbers. How much fuel would you have to save to pay for a second vehicle's insurance, repairs & depreciation, assuming you could double your fuel economy with an old Civic?
A buddy of mine keeps complaining about the cost of driving his (compact) pick-up. He wanted to buy my car. I told him if he did the math, he'd probably end up spending more money to buy mine than he'd save in fuel if he just keeps his truck.
Too bad we don't have a tool on the site that would help people make decisions like these. Right, Benjamin?