Smooth wheel covers can look good on some cars, might try the idea out?
Get her a scan gauge or ultra gauge, feedback is the key to changing behavior. that will show her when she is lower then epa.
also make a game/ bet about mpg, highest each month is the winner. The winner each month gets to pick the date spot and the looser pays for it
may or may not work but might be a way to make it fun and ease the tension.