speedo out

I just changed my chain and both sprockets, since then my spedo has been around approx. 20km/hr at realistically doing 100km/hr. Thats where it only reads 80. I had my gf follow me in her car and let me know. I was wondering if there was something I might have done to do this and how i can re-callibrate it. My bike is a 98 gsxr 600. thanks