DENVER, Colo. — There are certain assumptions people often make about diesels. They’re dirty. They get good fuel economy. They’re loud. The dirty part is no longer true thanks to stringent emissions standards in the United States. You can’t bring a diesel here that’s dirty, and if you cheat on your emissions testing, well, then you have Volkswagen. The fuel economy part definitely rings true – even in a pickup truck like the Ford F-150, which is estimated to get 30 mpg on the highway in the 4X2 configuration.