Your right, and therefore a nice card will be sent to HD thanking them for saving me all that money I would've spent on traffic fines
Correct!
No ones mentioned tyre pressures ! This also have a bearing on the rolling race of a vehicles tyre diameter, this can vary the diameter more than the wear on the tyres,2psi extra pressure may not seem a lot but can increase the size by a few thou.
As far as gps speed readings go I always take these with a pinch of salt, some satalite data takes split times and give you more of an average speed over time, I personally like the old fashioned mechanically fixed speedo that is on my bike that I kinda trust as opposed to some satalite system that supposed to tell you everything but nothing well!
If your not sure just split the difference between the two, but to be honest as you say big deal if the harley speedo runs a bit fast it's only to take into account all of these variables and best to be safe than sorry I say!
Oh and I will thank Mr Harley for saving me money on speeding fines just when he's skinning me alive with the prices they charge for servicing,parts,upgrades etc.........