whatever the % change in gearing... you will throw off you speedo by the same amount.
So if you go from a 16/46 (2.875) to a 16/47 (2.938) that's a 2.2% change so your speed will be off by that much. 150mph will display 153mph.
Aslo speedo's are off from the factory mainly ot correct for changes in tire circumfrence as it wears. The difference between a new tire and work out tire can be at least 3%. Addtionally, so tires have a taller profile. SO the manufactuires add a lot of paddding ot be sure it never reads slower than your going. It's kind of like how that 12 oz can of soda is really closer to 12.2 or 12.5 oz of actual fill, because there is no penalty for giving you too much, but big fines if you are shorted.