I have a 2013 FORD CMAX with 250K miles and I have been using flat LEXAN wheel covers for 240k miles and I gained 2 mpg which I have tested many times. I have read they increase 4 % mpg's.
https://fordcmaxhybridforum.com/uplo...1382541182.jpg My mpg average for the last 140k miles is 52.8 mpg.
Paul