Run a CR940 with a 6r. 185bu corn on flat ground, 6.5mph isn't hard. It'll do it. I only run 4.5 though, anyfaster the trucks can't keep up. Beans we run 4 with a 25 ft. I know a guy who has a 960 with a 30ft that runs as fast as his grain head will do. Sais it will do a good job up to 7.2mph when its dry and around 6.7 when its kinda tough. Hes in atleast 50bu beans.