77 speedometer doesn't work.
 

I bought my car a week ago and every gauge works except the speedometer. I have no idea what it could be. Can someone help me with this?

check the cable make sure its not busted

Could be the cable, could be the drive or driven gear in the trans, could be a faulty cruise control unit (if you have cruise), could be the speedo itself. Easiest way to check is to pop the cable off the back of the speedo, take the car for a drive, and see if the cable end is spinning.