You left way too much info out.

We need to know what fuel you are using, what kind of driving you do. Any recent tune ups done to the car?

Also don't forget the cold weather will reduce gas mileage and never follow the DIC 100%, do the math by hand and reset the avg mpg everytime and see how close it is to the real mpg.

And yes a bad EGR will affect mpgs on the freeway when it is active.