speedo

i know i got two probs on here but my car is in good nick, haha my speedo is reading wrong like for eg 70 = 100 if u do 90 = off the clock and its the fastest golf ever like 0-60 3 secs haha good to show mates a tosser for speed cams whats wrong!! standard wheels on

could be that the weight on the back of the speedo has come off? could be the tension spring? but I would imagine that a complete speedo replacement is needed you can't really repair these unless you have another to rob bits off?