For example, the index gives an American-made car no credit in the profit margin category if the carmaker is based overseas. With Ford Motor Co. and General Motors, DuBois argues, the majority of the company's shareholders are in the U.S., "and the profits go to the central coffers and are paid out" to mostly U.S. shareholders. With a foreign carmaker such as Hyundai, Porsche or Toyota, the profits primarily go back to shareholders in their home countries, he says.

If a carmaker assembles its vehicles in the U.S., however, the index grants it full marks in the "labor" category, as well as in the category of "inventory, capital and other expenses." Those scores reflect the wages the carmaker paid to American workers, the money it reinvested in U.S. manufacturing facilities and taxes it paid here, DuBois says.

So that's DuBois' formula. If you're looking for more detail, here's a deeper dive into where the money goes when a car buyer takes home an American-made car produced by a foreign carmaker.