To find the exact MPG for *your* 2009 Chevrolet Cobalt, I recommend checking:
* The fuel economy sticker: This sticker should be located inside the driver's side doorjamb.
* The owner's manual: This manual will list the EPA estimated fuel economy for your specific vehicle.
* The EPA's website (fueleconomy.gov): You can search for the fuel economy of your specific 2009 Chevrolet Cobalt by entering the year, make, and model, and optionally the engine size and transmission type.
Without knowing the specific engine and transmission, I cannot provide a definitive answer.