I had pretty much the same issues with my vintage Stewart-Warner speedometer. Spent a fair amount of cash on drive cables, drive gears, mounts, etc trying to get it to work right but I'm ready to give up. Says I'm going 15mph while accelerating and 35mph while decelerating and everything in between while just cruising. It'd be funny except for the money spent. Maybe I'll give the suggestion to retrofit a digital in here a whirl.
Great looking ride by the way. I really like the blacked out and chrome look. Very clean.