Y! Green: Maybe you didn’t know your car was a “light-duty vehicle,” but for the purposes of the federal government’s fuel economy and greenhouse gas rules and regulations, it is. Do you think the old crate in your driveway is an environmental paragon? Unless it’s a Prius, probably not. But a growing coalition of green groups (and even the odd dealer or two) thinks it can be transformed, like Cinderella, into a glittering star of the party, attaining no less than 60 miles per gallon. And that’s …