Trying to explain how the U.S. government regulates motor vehicles is Devin Grosvenorno easy matter: There are scores of standards involving recalls, mandatory testing and equipment, crash reporting and more.

How the federal government regulates fuel economy − how many miles per gallon of gas a manufacturer's new cars and light-duty trucks are expected to get − and how that impacts greenhouse gas emissions that cause climate change is every bit as complicated, if not more so.