I do a lot of highway with my 2014 Yaris CE and I wasn't pleased with the stock mpg.
I started with a few small things but once I made my own custom undertray, results really started to show. Now with my current setup, I have no problem maintaining 42-45mpg city and 50mpg highways. With some hypermiling, I've had my scan gauge hit 60mpg a few times now which was impossible when the car was stock.
The front grill and part of the bumper were also blocked off along with a rear diffuser (No fins just cover) and the gas tank. All I have left to cover are parts of the floors to complete the undertray.
High-speed stability has also improved dramatically along with side winds and road noise. Not the prettiest but its functional, secure, and cost me under 50$
