Mine gets 17 mpg driving around town and that is ave. But if your driving on the freeway then it's very bad. Check your fuel lines under the hood to make sure that they are not leaking on top of the supercharger at the connection to the fuel rail. I had this problem.
It also depends on the maintenance you have had performed. Fuel filter, tune up, air filter, oxygen sensor, oil change on a regular basis, what type of driving do you do, how hard do you accelerate, etc.