I'll use your thread as a segueway to offer one option. The example I am using is the butt of a Lexus GS :
I chose the Lexus because it already has nice "GS" lettering. In this case, the GS stands for "GasSavers". For your sedan, you could keep changing the numbers as you up your MPG. If you had a "GasSavers.org" sticker in your rear window, I think people would learn to get the "GS". I have a plastic hatch and alot of plastic behind the hatch, so changing the MPG numbers is hard for my wagon (but still doable, as I sit here thinking).
The key is finding a good set of (pre-existing?) 3D letters to put on the trunk. However, Honda Civics used to just be flat cheesy stickers (saved weight on the VX, right?
). so you *could* keep updating the numbers sticker, ala McDonalds.