The detail that would add some clarity to your BSFC map would be to mark the average Nm output of your engine. Then focus our attention on the part of the map a little above and a little below. If the load becomes moderate to high in 5th we already agree that a downshift is in order. Our difference is that I believe the engineers who design the trucks know when this downshift should take place better than we do and design the trucks to make the downshift at the appropriate time. You seem to think the rule is that if tow haul is needed 6th should be locked out. If conditions are such that the transmission is annoyingly shifting back and forth I would say that is one time I’d lock out 6th.
