My highway mileage has dropped from about 29 mpg to about 25 mpg. Same roads etc. City mileage is about the same over the same period, perhaps slightly lower. Any ideas why?

Keep in mind this car has had new plugs, oxygen sensor, knock sensors, air and fuel filter over the past 6 months. No codes shown with my peake tool.

I am puzzled. I am thinking partially due to ethanol in fuel. Other thoughts include leaky injectors, weak coils, transmission lockup issues, old fuel pump.