I normally run 4 to 4.5 mph in even 50 bu. beans with a 15 ft. head. loss out the back is acceptable to me. Of course, it would possibly be less at lower speeds but I will gladly sacrifice a few tenths of a bushel of $4 beans to get more done per hour. I usually only notice loss being a slowing factor when corn is 175 bu_ac or better with a 6-30 head.