IIRC there's some EU directive which says car speedos are allowed to over read by about 10% but are not permitted to under read at all. In older cars, I've been told that speedos may also start to get more and more inaccurate over time - although in more modern cars (which use electronic wizardry) this shouldn't happen. No idea if that's true or not.
Doesn't really answer your question though. On my Fiat, I'm pretty sure the speedo over-reads by about 5%.
