ok jeff, 540s were made 78-81,about 2900 made. 550s were made 78-88, little more than 6700 made. 15ft. would be max on a 540,as back end can`t handle that much straw,preferably 13ft.in good soybeans. 550s will take 18ft. in good beans,20ft. in wheat,no speed demon though in wheat.