You most likely need a new speedo sender, which is mounted on top of the transaxle...it is most often responsible for erratic, or non function. Never heard of a consistant deviation, but would still think it is the sender. About $130 or so.
A Ferrari speedometer reading lower than the speed you actually are going is a bit odd, and would agree with Dave tht the likely culprit is the sender.
It is, however, quite common for Ferrari speedometers to read high, i.e., the gauge will show 77 mph when you are in fact doing 70. In many transverse V-8s that are geared at or near 20mph/1000rpm in 5th, the old standby is to approximately double the tach reading for a more accurate speed (e.g. 35 hundred rpm = 70 mph). A consistant 10% high reading error is not uncommon.
I wish someone could develop some vernier adjuster to plug in in-line between the sender and speedometer where we could calibrate our speedometers ourselves, much like a mechanical watch.