Good question. I too am like you and also have same bike - 2013 FLSTC. It's not like anything I've had before. I've not paid much attention to what speed I change gear at, I just listen to engine and feel the vibes. Sometimes I find I'm doing 45mph in 6th gear. This bike pulls in any gear but I try to change up quickly to keep below 2000-2500rpm so that the ride is nice and smooth.
When I go over 2500 the vibes start getting bad and I can't see anything in mirrors. At 70-80mph in 6th gear the vibes are so bad it will rattle bolts loose! Dealer said thats why they put thread-lock on the bolts! There's no rubber mounts, so I assume that's correct. One thing I have a problem with is downshifting, sometimes it just sticks in 3rd or 4th gear and refuses to shift down. I think it will all get better with age and with more miles on the clock.