If your MPG went up after the repair, its because you were actually burning gasoline instead of throwing it down the drain, literally.
I would check the spark plugs your using, stay away from those Bosch ones, they will burn up your coils, you can get the cheapest crap you can find an be better than Bosch, I would recommend the ACDelco ones.
If your coils were installed wrong, you would know about it, there probably fine.
What is the new code?
Old Car: Chevy Cobalt 2006 LS Sedan base (Automatic)
New Car: Chevy Impala 2011 LT Fully Loaded (Automatic)