The original question was that what counts as lugging (or mis-use, if you like it better). Your note about the engine weight sounds a good food for thought, but I still don't know where is the range of lugging
I still don't know anything better than my empirical approach: when it sounds strange, it's bad
But, I must admit, I don't have an exact definition for 'strange', not even with one particular bike
And I still don't know how bad is bad.
There are members at f650.com who even advise not to use this single under 4000 rpm - and I'm very sure they're wrong. If they were right, Teresa wouldn't purr like a healthy kitten after more than a hundred thousand kilometers.
On a side note, they also fear coasting, especially in a turn. They cross themselves when they hear about using the kill switch that's right at their right hand. They fear bikes that aren't faster than anything around them. They seem as superstitious as a sailor to me
And please don't blame me for choosing this 650. My original intent was to get a 250, but I just couldn't get a nice, quite new one (it was scary how absent (almost) new 250s were in Hungary in 2008!). My 'plan B' was a CBR125 back then, and if there's such thing as a too small motorcycle, that could have been one in hindsight. Not that I'm in love with speed and acceleration (I simply don't care about them), but because of the tasks Teresa had to do in the 3.5 years since I bought her. Sometimes she was literally used as a freight bike
What is a small mid-size bike for others (yes, this is the common opinion of a 50hp 650 among many bikers here) may be a heavy duty workhorse for a hypermiler. So I went with "plan C" in the end.
This choice was preceded by a lot of research on my behalf. I wanted to make sure I buy a (n only) vehicle with good fuel economy, and I found that this 650 is basically a 250 in this respect. My later experience that her appetite is very much the same as of our (carbed) Hyosung GV250.
Of course there are better 250s, just have a look at sendler's new CBR (now that he's into the tricks I utilize too, he's steadily well in the 90s with it, even though summer hasn't arrived yet!) - but this is one of the new models that did not exist back then. If I chose a bike now, that would be a worthy candidate, I could even buy it with ABS, which is absolutely worth its price. I'm also very curious about the NC700, which is larger again, but fits my "heavy duty" line (it's also regretfully heavier than Teresa, though). And the fuel injected F650s (now they're called G650s) are still in the picture for the same reason. I've heard about a guy who won an economy run with a new G650GS. With a pillow to help him tuck all the way, but he did 103mpg with his bike, a bit better than my personal best tank (98.9mpgUS).
p.s. I'd still like to have a gray/orange CBR125 (this new design is simply awesome), but we need the money for more important things. If we'll have enough extra to pay for it in cash before the end of the oil age, we'll probably get one...