The U.S. Department of Energy runs a website (FuelEconomy.gov) devoted entirely the fuel economy of cars sold in the US. It has the fuel efficiency of every model car made since the year 2000.

Not only that, but it gives you energy and environmental impact ratings, and fuel-saving tips. Heres a more complete list of what the website offers, directly from their About Us page:

EPA fuel economy ratings for passenger cars and trucks 1985-present; User-provided, real-world fuel economy estimates; Energy impact scores (petroleum consumption); Fuel economics; Greenhouse gas and air pollution ratings; Lists of vehicles that can use alternative fuels (e.g., E85, natural gas, propane, electricity); Links to fuel prices and crash ratings; Tax incentives for hybrids and alternative fuel vehicles; Driving & vehicle maintenance tips; And a downloadable Fuel Economy Guide.

It’s a rad site, you can do side by side comparisons and all sorts of searches.

Check it:
http://www.FuelEconomy.gov