The trip meter gears are held inside a small carrier. They don't really have a place to slide, so I'd guess some resistance is building up in there. When I put mine back together it wouldn't spin past 0.9 because there was some friction in there (I didn't put the flange back on the face). You really have to take it apart to see what's happening.
When removing a speedo needle, you first lift the end over the peg and let it rest. Reinstall in that same place, and lift back over the peg.
The spindle has a very delicate spring on it, and it is a very small spindle itself. Be very careful to pry straight up... mask off the gauge face somehow and get under both sides of the needle.
As for calibration... I eyeballed the position of my needle on reinstallation. Before I put the glass back on the gauge, I very gently chucked the brass cable fitting (where the speedo cable attaches) in a cordless drill and spun it COUNTERCLOCKWISE. The drill on full speed was around 1800 rpm and came in at around 60mph. I ran it for 1 minute and checked it to the trip meter. Did the math and it was quite accurate.
Once back on the bike I checked the trip meter against a measured mile and it read spot on.
