My 2005 3.0d's OBC fuel consumption is pretty much spot on with the manually calculated figure. Mind you, filling a diesel to exactly to the same spot each time is difficult as the diesel fuel foams up so much so you have to be really patient to get it right.

With regards the speedo reading fast, this is common for most cars. In my experience Toyota speedos are amongst the more accurate while Alfas are amongst the worst. I presume car manufacturers do this so as to avoid problems with speeding fines were you think you are actually going slower than you are.