Putting another GPS (I also have a Garmin 2610) on the handlebar quickly shows that Honda didn't change any of their engineering for the speedometer. My speedo is about 5% faster than reality which seems to be about typical for a Honda speedo.
With my 06 it seems to have about a 2-3 mph error. Earlier models seem to have a little higher error. Speedemeter is still off the front wheel I believe, not sure and don't care I'm guilty of assuming that I can always add 5mphs to Highway speeds. Tom
The Honda Navi does not have any input into the speedometer system of the bike. They are totally separate. There is a published list by manufacturer of each one's speedometer error somewhere on this board. Can't remember exactly where it was, but the Goldwing had the most correct. With standard 60 series tires it was an error of 4.1 mph faster than indicated at 60 mph I think. Most bikes were 5 mph or more faster. Tom Finch has reported that 70 series tires will make your speedometer accurate. He manufacturers the Tailwind trailer and can be found on the trailer board.