Thats pretty normal at least on an S2. The good news is your speedo is, actually, basically working. The speedos don't go down to zero, there is a very fine pin that stops it. Is it going down as far as the stop pin or not? If its on the pin then that aspect is fine.
If its a mechanical speedo those older speedos were not particularly accurate so although 12MPH is quite a lot it I would guess there are plenty out there more inaccurate than that.
For no cost, again if its a mechanical speedo you can strip down the speedo carefully and check the workings. If the needle is binding on something apply a little light oil to free it off.
Might be worth putting a new speedo cable in, though they usually signify their wear or lack of lubrication by a waving needle.
You can pay to have your speedo rebuilt / recalibrated, but it ain't cheap.
If its an electronic speedo thats way too new technology for me to give you advice on.
I suspect your speedo is electronic, going by your car details on the forum. I'd better say this quietly, but go to Alfa Romeo Bulletin Board & Forums which is the USA Alfa equivalent of this site. There are a number of threads there for both repairing and calibrating both speedometers and tachometers. Go to the Spider forum, then search on 'speedo' or 'speedometer' and see what turns up. I'm in there somewhere, because two weeks ago my S3 speedo was in pieces due to it not working!
Basically, there is a potentiometer in the speedo which is set at the factory and has a rubber type glue added to hold it in place. I think that over time or in a harsher environment, the connection on the pot deteriorates and causes a bad connection which alters the calibration. The general concensus seems to be that you should check this over and 'remember' the position, spray it with contact cleaner, give the pot a few twists with a screwdriver and then put it back in the same position with a blob of silicone to keep it there.
So far, this is just the easy bit......... The hard part is getting the speedo out in the first place! Don't take the dashboard out yet though, because if you are careful there is just room to remove the speedo assembly without taking the pod off the dashboard. Putting it back in is like fitting a tyre, so be careful since the plastic bezel is quite fragile.
To me, the big headache seems to be that to calibrate the speedo, all you can do is to adjust it, put it back together and try it. If it isn't correct, start over again. One US forum contributor claimed he calibrated his car over a 117 mile drive..........
If you can't find the info, send me a pm and I'll send the link. Better still, I've just found it.....