My 2008 Chrysler 300 (Touring) is only getting.

My 2008 Chrysler 300 (Touring) is only getting 11 mpg. There is no check engine light on -- or any warning lights for that matter. I recently had new spark plugs and a new air filter installed. I checked the tire pressure and changed the oil as well Yet I'm still getting only 11 mpg. Has anyone gone through this with their car? Did you find a way to get better gas mileage?