speedometer does not work!!!!!!

Hi, I have a 98 Civic standard. The speedometer does not work, and lights flick regularly. the output of alternator ranges from 12v(when press the gas padal) to 13v (when leave it off). When I start the car, the battery checking light was not on.
Is the problem from the speed sensor or from the alternator? Can the alternator failure cause the speedometer problem?

I got it checked today, they said that is the alternator problem, because when press the gas, the output was too low, only 12v. but can the alternator influnces speedometer? I notice that it is ususally at 0, but when I speed up, it goes crazy, like 200 something....... I am so confused, anyone has any idea about it?