epa.gov has posted info discussing everything about its new mpg rating tests. Basic info can be found at
http://www.epa.gov/fueleconomy/basicinformation.htm
which has a graphic of the new EPA fuel economy label for all new cars and light trucks.

The draft of the EPA fuel economy tests can be found here
http://www.epa.gov/fueleconomy/420d06002.pdf

Other EPA fuel economy and emissions programs info can be found here
http://www.epa.gov/fueleconomy/420f04053.htm