What is this on this gage?
 

I have seen these and was wondering what they are and what they are used for.

Thats a vehicle speed sensor. It just counts the number of speedometer cable rotations, and doubles it to make a 2 pulse per speedometer cable rotation signal that the ECM, and cruise use :)

Thanks, that is not my gage in picture. I took it off of the internet. The reason I asked is because when I took my gage out there is one of these hanging loose and was not connected to my gage anywhere. Is this suppose to hook onto the gage somewhere? Is this something I should have hooked up? I just want to make sure I have everthing right when I do put back together.

As Russell stated it is the Vehicle Speed Sensor. It is the optical sensor for cruise control. Here is a thread on what it looks like hooked up under the dash:

http://67-72chevytrucks.com/vboard/s....php3?t=167510