Poor Fuel Economy on 2010 Chevrolet Aveo5

My 2010 Aveo5 has 13000 miles on it and has seen a drop from approximately 26-32 mpg to 20-23 mpg any idea what could be causing this problem? The dealership says they cannot find anything wrong.

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2 answers
Are the roads smooth or are they snowy, because muddy, slushy driving conditions will cause a drop in fuel mileage.
Well fuel mileage has been poor for a few months. Conditions have veried as have the type of driving. Ie highway, city, or suburbia. I have also put a new k&n air filter to try to fix te problem but have had no luck.