I have a 2009 Chevy Impala. Since Nov. My gas mileage has been averaging between 22-24 mpg. I took it to the dealership and they say nothing is wrong. The car was originally titled in Hawaii. Its not showing any codes.
Honestly, gas mileage depends on too many factors to really "depend" on what the EPA says it should be. The 3.5-liter V-6 is rated at 18 mpg in the city and 29 mpg on the highway and the 3.9-liter is 17 mpg in the city and 27 mpg on the highway. So your 22 to 24 mpg isn't unrealistic, especially if your driving style has changed. On a side note, is the Impala a Flexible Fuel Vehicle? If so and you are running E85 gasoline, this sharply decreases your fuel economy.