Thanks for this...Lets go back to you earlier readings with the oxygen sensors, start looking for a vacuum leak. Put a vacuum gauge on the intake and see what the reading is, Try switching the sensors to the opposite side. The other thing that is worth checking is to pull the plugs and re-inspecting, they will show either rich or lean. Rich or lean both will ruin fuel economy.
Some of the manufactures have been quietly known to adjust the speedo to run out the warranty sooner. But that opens up a discussion that does not pertain to mileage issues. Just thought I would throw that in for fun.
Remember KISS....Keep it simple and start at the basics, What effects fuel air ratio if none of the sensors or codes show and one of the main ones I have commonly seen is unmetered air on the intake side.
Here's some thoughts...
1) I don't have vacuum gauges.
2) I read someone who is much smarter with engines than I that he replaced (and recommends replacing) ALL the vacuum hoses in this motorbike. At the same time, if I do this myself, I could pull the 2 or 3 5-way tees and ensure their cleanliness.
3) I intend to pull the plugs to check and see if one is running rich. How do I tell if one is running lean...??
4) How easy or hard is it to remove the O2 sensors...?? Removing them and having a visual inspection should help me assess if one (or both) are getting terribly clogged with carbon build-up.
5) Since it seems as though the ST13 standard speedo/odo runs so the speedo shows higher than actual speed, but the odo reads close to true, mine with a "Speedo Healer" runs pretty close to perfect for speed, and about 6 percent short on km. So if I drive 400km on my odo, I'm really driving 424km. But if my speedo shows 100kph, I'm going 100kph. The speed is corroborated with street-side radar speed indicators, live GPS on my bike (cell phone), and with the police... (you likely don't have to ask me about this last one... lol)
When I do my fuel economy calculations, I take #5 above into account.