just got my 87 gt back from my mechanics who fixed my dash and put in a roll cage.
half way through the 1 hour drive, i got bored and broke 110mph, started backing off, look down and the spedo just says 0
i tryed opening up the dash, the only thing that is plugged into my AutoMeter gauge is the spedo cable and the light.

havent been able to check under the car yet
any ideas?

 

Yours is a gear driven cable. I would inspect the cable to make sure it did not get damaged or pinched. You can undo the cable from under the tranny yank out the core wire inside and make sure its intack. On my SVO it snapped since the speedo cable had a gash.