hi tomp,
sorry it took a while to get back to this, but I'm going on holidays and was wrapping things up.
The main reason for your Sion and your C14 having the same power but different top speed is simply aerodynamic drag. Rolling drag is much, much smaller, but not negligible (well, that depends on the level of OCD
). As a rule of thumb, for B-road speeds, rolling resistance is about 10% of the aerodynamic drag, going down to 5% at highway speeds. For a really good overview of this, please check this
link.
The fuel consumptions reflect what I've seen in my bike, meaning 60 mpg km at 60mph and around 20 mpg at twice that speed. And 12 mpg at 155 mph.
Keep in mind that motorcycles have a really bad Cd compared to cars. In fact, until some years ago the queen of Cd was the Busa, with a Cd of 0,47, whereas the "normal" bike has 0.6 or worse, which is that of a delivery truck
Same with rolling resistance. Also, the rolling resistance changes with speed. For example, you can see
here (page 42, the graph with the tire at 42 psi/2.9 bar) that the rolling resistance at 30 mph/50 kph is just 0.015 and it increases to 0.025 at 3 times that speed. I guesstimate 0.03 at 125 mph/200 kph.
Just to hammer the concept a bit further, at low speed (under 60 mph) the mass will resist acceleration more than aero drag, but at 150 mph the aero drag takes most of the effort to overcome. Kinetic energy seems tempting to use here, but the rate at which it changes is not that high, so really, aero drag is the key. Still, as datsaxman said, weight has little to do with top speed. And I mean it literally:
little, but
not nothing.
EDIT: In fact, at the bottom of the page 42 on the book from the link above, there's an example in which a motorcycle uses as much as 12 kW (that's a whooping 16 HP) to overcome rolling resistance at 250 kph/155 mph. That's 12% of the C14's max output at the rear wheel (assuming 135 RWHP). Power requirement increases with the cube of the speed, so if we could somehow eliminate the rolling resistance and get back those 16 HP and use them to overcome aero drag, we would get a top speed increase of just 4.3%. Now, I put my bike to 167 mph, so that 4.3% would add a miserable 7 mph, taking the top speed to 174 mph. I'll live.