45mph is extremely low for 6th gear, you should be in 5th unless you are coasting down a hill. The prompt is only there for best mpgs, its not the rule and in most cases ignored. Any power in 6th gear at <50mph puts unnecessary stress on the drivetrain and your engines main bearings. That would include trying to remain at 43mph in 6th on a slight incline. Anything less than a cruising speed of 55mph I drop to 5th gear. You do it for the same reason you don't try to do a full throttle pull in 6th from 60mph. Yes, an auto probably would shift into 6th at 43mph, but you'd then soon see it downshift to 5th after you apply some power. On hilly roads around where I live, this aspect of autos drive me nuts...constant never ending shifting, ultimately hurting mpgs and making me mad!