I just got back from a duathlon a couple hours away (according to the Scangauge, the car averaged 65.4 mpg on the way down, and 66.7 mpg back), where the signs in my windows were a hit. These signs:
I noticed several people stop to read them, and two people asked me about the wheel covers. Maybe something will "catch," and someone will start thinking more critically about their vehicle's design. Since the triathlon crowd is already concerned with aerodynamics, it shouldn't be a huge leap.