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.

Asked by for the 2010 Chevrolet Aveo5
Are the roads smooth or are they snowy, because muddy, slushy driving conditions will cause a drop in fuel mileage.
1 more answer
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.