Tafi,
For starters, it would help if you mentioned what year spider you have. Earlier ones, like mine, have mechanical gauges. Newer models rely on electronics.
The best I can do is direct you to the websites I found, but be warned: I have no idea if the information presented by them is accurate or if it will be helpful. I'm putting off my repair until fall/winter, so all I've done is take a very cursory look at the sites. Also I was looking for information on Jaeger gauges. If yours is different, I doubt these will do you much good.
http://hem.passagen.se/veloce/MECH3.HTM
http://www.alfaclub.org/techstff/speed.htm
http://mywebpages.comcast.net/rhodes/speedo.html -- this one looks to be the most comprehensive, but the writer's experience is with Triumphs not Alfas. Still I'm hoping to be able to apply most of the techniques to my speedometer.
Good luck with your repair.
--Toronto