If you're talking about just replacing a needle you removed, probably the best way to reattach it would be to use a laptop-based OBD-II setup. Drive at a set speed, get the reading from the VSS, and pin the needle at that speed (make sure you have a friend help you so you're not endangering yourself or others, or use a dyno). I had to do this when switching from red to clear needles, and when I put the needle back on it was dead-on with the VSS. Previously my speedo was two or three mph off compared with the computer.
If you're talking about the VSS (and thusly the speedo) being off (like when you change wheel/tire sizes) the only way I know of to recalibrate is by messing with the computer; something I don't really know anything about.
