If you think your mileage may vary with a brand new vehicle, take a look at the differences in fuel economy results for older cars. Sites like the EPA's Fuel Economy tracker and Fuelly prove that there is not such thing as a standard MPG number, despite the EPA's best efforts. This all isn't stopping the EPA from releasing a new window sticker designed for used cars, which the agency announced today. It could be an important tool, since over 40 million used cars were sold in the US last year.