Maximum Cruising RPM

[Bobn]

This has probably been asked before but I never have any success with the forum search tools…
What RPM do we consider to be the maximum for sustained motorway cruising on these bikes?
On a 15T front sprocket, 70MPH (indicated) is 4600ish and 80 is just under 5000 but It’s all starting to feel a bit fraught at 5000 to be honest.
The redline looks like 6750 (is that right) and I seem to remember an old adage about not running an engine at more than 2/3 maximum RPM for any length of time. 2/3 of 6750 is 4499 RPM so that should mean that 70 MPH is fine and you can take a blast up to 80MPH for getting round stuff.
I always try to feel and hear what an engine is telling me and mine seem very happy at 4000 to 4800, after which the vibes increase suddenly (which I take as a sign to calm it down), also peak torque is 4500ish so that means the engine is at its most efficient at that speed.
Any thoughts?
BTW I went to France last year at 70MPH the whole way, could have gone faster (I was riding with bigger bikes) but I set the speed and they had to lump it. The FX did 800 miles on that trip and seemed to love it.

[Rubberchicken]

Mine was always a bit dodgy on motorways, but then it had something wrong with the valve seals and would start boozing oil when (not if) it ran hot.

It seems happiest keeping things below 5000 rpm, indeed. Though I'm not sure if it's really a matter of rpm per se as much as operating temperature and ability to shed heat. It's an unregulated aircooled lump after all...

My advice: Get one of those dipstick oil temperature gauges. Gericke used to carry the "RR" brand which is good. Cheaper ones can be found but there's a reason...

With that in place, you can get some feedback on what the engine is doing and when it gets too hot. Get a feel for what's going on in there and then you'll know when it's time to back off a bit. On mine, I needed to keep the oil below 115 degrees C or I'd lose it.

[Bobn]

I've looked at those temperature sticks - they're a bit pricey.
I might be more tempted to put a thermocouple in the tank and an LCD readout on the dash. The subject has come up before and I still think that given my personality type I'm better off not knowing the oil temperature - I'd just be looking at it and worrying the whole time!

On quiet pipes and original jetting the FX seems happy enough up to 75MPH, with open pipes and 175 main jet I find I actually go more slowly, the noise at 66MPH and 4300 RPM is glorious! It's also easier on the upper body. When I went to France I did 70-75MPH the whole way, that was on open pipes with 175 main, but I'd fitted a windscreen and throttle lock which hugely increased rider comfort - I don't think I'd have made it stuck out in the wind!