I shift when it needs it,,, different circumstances need different gears! Use your experience to shift, He!! remember when no Harley had a tach? You had to use your ear!
If the manual says it's ok to be in 6th at 55, I guess it's ok. I try to keep the tach at a minimum of 2,000 rpm's. 55 mph in OD isn't bad on smooth and flat road.
I'll do it if I can maintain a steady speed on a flat road but I'll drop down to 5th if I approach any hills or if I have to accelerate for any reason.
Someday I'm going to have to add a tach so I can see actual engine speed.
I can shift at 55 with no problem. Acceleration is a little sluggish but no lugging,knocking or pinging. I usually shift at 60 because acceleration is better. This is on a flat road. If climbing hills then whatever it takes to maintain 2500 rpm. Shifting through the gears at 2500 comes out to about 20,30,40,50,60 mph, from what I remember. About the only time I'm over 3000 rpm is when I miss a gear.
I guess I may get over 3000 on the interstate when I open it up a little bit-don't remember.
I don't pay a whole lot of attention to the tach on my 09 but I think 60mph in 5th is about 2400rpm? Looking at Tq charts, this is about where the good torque is picking up. Wouldn't this be the ideal rpm range for pick-up and mpg?