In the fiercely competitive auto industry , manufacturers look for every means of improving efficiency . One method is to seek economies of scale by building ever bigger factories . These vast assembly plants , occupying millions of square feet and producing ever more vehicles , still employ thousands of workers despite intensive automation . Here are the 10 biggest US auto plants based on 2015 ’ s figures , the most recent available . Where possible , 2016 production rates are cited to give an indication of what the future holds for America ’ s biggest car plants .