Medium- and heavy-duty trucks comprise 4 percent of the total vehicles on the road, but account for 20 percent of the fuel consumed and 20 percent of the greenhouse gases (GHG) emitted by the transportation sector. Now, for the first-time ever, fuel economy standards for heavy- and medium-duty trucks over 8,500 lbs. will be regulated by the federal government. Here are the pros and cons of this new mandate.