91 Escort Pony Speedometer HELP!

I have a 1991 Ford Escort Pony. Recentley while driving on the freeway the speedometer shot almost all the way around, telling me I am going 80mph while really doing about 50mph. It is making a wierd sort of almost buzzing noise, and the needle moves wildy about. Has anyone heard of this with escorts, or could this be something major? The car seems to be running pretty normal, I haven't heard any wierd noises or smelt any burning. PLEASE SOMEBODY HELP ME...I AM NERVOUS ABOUT THIS...HELP! Thank you very much for your time!!!!!!!!![/COLOR][/FONT][/FONT]