After a blockbuster year for new car sales in 2015, growth in the auto industry has slowed in 2016. U.S. sales in August for General Motors and Ford were down 5.2% and 8.4%, respectively, compared with the same month in 2015. But automakers aren’t accepting the downturn passively: to juice sales in September, incentive spending is at the highest levels ever. According to a J.D.