Hey guys,

I have an issue with the speedometer in which the needle bounces under acceleration or even going over bumps. Under hard throttle it really starts going crazy, up and down by 10 to 15 mph until I maintain a constant speed.
If I am cruising at a continuous speed it settles down and stays fairly still. I have tried a new speedometer cable, a new driven gear, and the T5Z has a new drive gear. I also tried another speedometer and still have the problem. What gives?

 

well if I accellerate hard the speedo needle sticks to the glass and doesn't move until I shift. Newtons first law seems to apply to my car as well :-)

no idea about the 66 speedos though, I have the Falcon long type (early model)

 

You must have one fast stang!!!

 

Your speedo cable is probably kinked or the internal cabling is frayed. Try graphite dry lube at both ends or replace the cable. Also, secure it away from any heat sources.