EPA's Green Vehicle Guide gives information about the environmental performance of vehicles. http://www.epa.gov/greenvehicles/
The Green Vehicle Guide uses a 0-10 emissions score and the fuel economy numbers to compare environmental performance across vehicle classes. Larger numbers, whether emissions score or fuel economy, are more environmentally friendly than smaller numbers.
The Fuel Economy Guide lists the city and highway fuel economy estimates for all cars and light trucks as well as estimated annual fuel costs. There are also tips for improving fuel economy.