speedo gone crazy

my girlfriends 2004 cavalier had a little hickup the other day on her way home. Everything was going fine then out of nowhere the speedo is going 220 km/h (the max on the speedo). of course she is not travaling at 220km/h so i told her to just drive at 2000 rpm until she got home. She has faith in me that i can fix it when honestly i dont have a snif as to what could be wrong. i was reading on some other threads and they seem to all have problems with more than one gage this is just the speedo, all other gages are working fine. so if anyone could help that would be super.

Well it could be the speedo or the speed sensor.

Actually all we did was disconect the battary for about an hour and connected it again. She has been driving for about two weeks now and no problems so far