I haven't been able to find the one I first saw for motorcycles, there is a compeny that makes signs that bolt on above or below your lisence plate that come in "40mpg" "50mpg" and "60mpg" I think they might have said "and having more fun then you" or something like that.
but
mpgstick.com sells them from 30+ to 80+ mpg as bumper stickers.
CafePress also has mpg stickers ranging from
up to
in 1mpg incriments