The Used Car Label Tool, found at fueleconomy.gov, generates mpg and environmental information based on the year, make, model and certain options for cars dating back to 1984.
Printing one of these labels and sticking it on a used car may not necessarily help you sell it. In 2008, any models dating before then had its original fuel economy rating downgraded by two or three miles.
"The older test was biased a little high," Doug Greenhaus, chief counsel for environment, health, and safety at the National Automobile Dealers Association told The National Journal. "In almost every instance, [the new test] is a lowering."
The number on the sticker of a used car should be taken with a grain of salt. Depending on how the maintenance history, used cars often perform less efficiently than new cars. The sticker includes a warning, which states "actual fuel economy will vary for many reasons including driving conditions and how the car was driven, maintained or modified."
It's speculated that the stickers will be used mainly on fuel-efficient used cars, because there is no regulation requiring used cars lots to post the stickers in every car.