I just got a new to me 2015 Limited, I rode a 2006 Road King and I must say the difference in rides is amazing.
Not used to 6 gear, and was wondering what rpm the motor is happiest in and what speed do you use 6th gear. do I keep the bike in 5th gear at highway speeds below 65, or do I use the tach as a better gage of what gear to be in,

 

I've used mine on my 08 ultra with a 103 and my 14 Limited at 55 in the flat no problem. You do not want to lug the engine. Always remember to downshift before you labor the engine. If your not worried about the mpg, it's better to use it above 65. I could get 53mpg on my 08, the 14 not so much.

 

If I am crusing backroads and holding 60 or above I will set the cruise in 6th gear and let it roll. It has plenty of pull at 60 and will roll on just fine.

 

Its mostly a preference, the manual says you can shift to 6th at 55mph. Instead of following a specific procedure, I ride the bike where it feels good at the time. The bike will let you know if it does not like you.

 

I don't use 6th in my 10 or my 16 below 70.

 

Same with me...almost never below 75 really, and in 5th at 70mph, im about 4grand on the tach as I recall. If i ride 6th before 70/75 mph, it lugs and lacks any real pull upon snapping of the throttle.

 

I have ridden with 5 speeds and 6 speeds. And everything I have always heard, it run the bike at about 3000 rpm. On the highway of course. I have a friend that's a HD tech. He swears by that. Says Harleys like RPM. Just my opinion.

 

In 6th Gear, nothing under 2000 RPM's, my 14 SG likes to
run 6th gear between 2500 to 3500 out on the open road.

 

Anything over 55 for me if I'm just putting around. I can still roll the throttle at that speed, but don't come on to it hard. If there's much wind, uphill, or if I'm riding aggressively and want instant throttle, then I'll let it rev in 5th. It's really hard to say that at x rpm you should be in a certain gear. You'll bike will let you know. Also, what you have done to the bike makes a big difference IMO. If you are stage one or better I find 6th gear much more useful. I had an 09 softail that when stock couldn't really pull bellow 70 in 6th, the new limited with stage one is a different story for sure.

 

After 1st gear I keep my RPM above 2200