The speedometer stays at 0km until you're going about 30km/h. Then it will start moving. The needle doesn't jump around at all, but it rarely reads the right speed, the odd time it does. When it reads 40 your going about 60km, and when it reads 120 your really going about 100.
All of the other guages work properly. Would this be the VSS?

Probably. It is also the easiest place to start. And being a mechanical device the most likely to fail. Is your shifting off also?