good for the economy? as in expensive? -or did you mean good for the environment? as in a small carbon footprint? also, are you looking for something new or used? jeremy's right, the volt get's my pick for new car. it uses a gas generator to power an electric drivetrain, rather than a combination of a gas and electric drivetrain. fewer expensive parts to break, if you ask me. plus, a little less complex. train locomotives have been using that setup for 50 years. it works for them....and the first 45 miles it runs off the batteries. if you're looking for older cars, 80's hondas. civic, accord, crx. great mileage. good luck.
Chevy volt is coming soon...also the Tesla car (the current one is small, sports car) but the next one is coming out in a year and all electric, so in some ways, even better for the environment.
Also there is the Nissan Leaf, they've been putting that one in a lot of ads on TV and it apparently won an award in Europe.
any car in general from honda, doesnt matter if its an L4 or a V6, there are some smaller fords but those aren't very safe... almost anything with a 4 cylindar engine is good on gas.
Toyota Carolla is good, it's small, and runs on the smell of an oily rag! Haha.
Hondai Sonata, mine is great