They sell decals on the site for $5, but I like your idea of adding each members MPG somehow.
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While I was working on my undergraduate senior project (mpg computer similar to MPGuino) I thought about mounting a small display in the rear window that showed my instant MPG. But I'm not sure if that is safe for other drivers, and I'm not sure of how legal it is to have a display showing out of your window. But you are correct, advertising our MPG numbers is a lot more interesting than just the website link.