My bike has been sitting for a while and when I finally managed to start her up i'm experiencing an odd fault with the speedometer/tachometer that I was hoping someone would be able to help with?

When the bike is reving the rev counter works fine but the speedometer seems to follow the rev counter when the bike is stationary! For example if i rev to say 3000 rpm the speedometer reads around 30 MPH, I know this will be electrical but I dont know where to start.

Thanks
Brian