Presto writes: "It seems that most of these answers are along the line that it's the rider’s subjective response that determines cruising speed – an engine might seem stressed to the rider when objectively it may not be stressed."
Absolutely spot on. Anyone who thinks they can tell whether the engine is "stressed" is living in a dream world. The engine is just steel and aluminium which obeys the laws of physics, and the stress and strain levels can all be calculated mathematically; some can be measured empirically; and none can be determined by a rider which his bum on the seat and his hands on the bars.
The only data we can work with is the engine rpm. We know the stress increases as the engine speed increases; what a rider cannot tell is whether the stress exceeds the manufacturer's calculated maximum for each component in the engine.
The only other guideline we could use is statistical data on engine failures. Unfortunately, whilst we have plenty of stories, nobody has gathered enough statistically valid data in controlled circumstances to state categorically the maximum safe speed a Bullet engine will sustain indefinitely.
In the absence of data we (being humans) form opinions instead, and (being human) we defend those opinions like we would defend our own person. That is why everyone has their own opinion about the optimum cruising speed and it usually differs from everyone else's.
Of course, I'm going to join in, as I'm human, too! So my uninformed opinion is this: 55 - 60 mph for a sustained cruising speed. I should reveal that I'm cheating: my '02 iron barrel Bullet came with a slip of paper stating exactly that, and emphasising its unsuitability for modern high speed roads.
