20 mpg as an average would be nice for a hemi, 6 speed. I can get 20 mpg on the freeway on mostly flat no wind days and being very attentive to the Fuel Saver icon. But an average depending on the mix will be around 15 to 17 mpg.
Lately I have been doing about 90% city and have been averaging right about 13 to 14. If I go out in rush hour traffic subtract 1 mpg.
A lot depends on your driving habits, how heavy your foot is and how much coasting you can do before hitting the brakes. The biggest part of fuel consumption in city driving is starting this brick from a dead stop. If you have any momentum between traffic lights it will use less fuel to accelerate.