You are right on about the speedo error from the gearing change.
You bike is traveling 3.8 % further than your odo indicates, and 3.8 % faster than your speedo indicates.
But that doesn't explain the other 3/4s of the 13. 5%+/- loss of fuel economy.
Hmmm, maybe you are spending more time at bigger throttle openings than you think you are!
That would easily explain the other 3/4s of the loss.
Plus, you just changed your rear tire, correct?
Is your new tire larger in OD than what you took off?
That might make another 1 or 2 % difference.
Surely whatever the loss is from rests within the above, as it's been running nice and consistent for you all along, and you've not done any other work that could affect such things, correct?
I've changed nothing else and the tire is the "same size" not counting for 10 years of wear on the old rear and different brand.
The "seat of the pants" dyno says a loss of torque and I've never been WOT on this bike before the gear change.
So I think I'm actually getting less MPG because of my riding style. I was never a WOT rider, but was aggressive and even more after the new tires because they were like glue compared to the old ones.
I'm thinking that my riding style would do better with a 16 front or I have to learn to be less aggressive. I'd probably be best off if I backed off a bit, but I'm having a lot of fun with these new tires. I really do enjoy the blast of torque right in the middle, it's just damn fun and now I have to go WOT to get what I had before. Strange that only a 3.8% change would make that much difference.
The 16T front would increase torque by 5.9% or 2.3% more torque than stock. I can't see 2.3% being that much, it would be damn close to stock, but would I actually be getting better MPG?
~15% really can't be just the speedo correction alone.
Does the PC III do any speedo correction or is it just fuel? I'm asking because the best setup might be a 16 sprocket and a PC III to get the smooth torque at the start. The 17 sprocket does fix the jerky part down low.