Temporary incentive induced bump up in 2017
https://www.google.com/url?sa=i&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwiTvJCtsKfgAhXH5YMKHZ6aAUwQjRx6BAgBEAU&url=https%3A%2F%2Fwww.statista.com%2Fstatistics%2F239607%2Fvehicle-sales-market-share-of-general-motors-in-the-united-states%2F&psig=AOvVaw3uxe_Kg1w2_TNjRfiM-Y8O&ust=1549551605404013