Check the electronic speed sensor that is located on the side of your transmission. seeming that the rest of your gauges work (and the speedometer on our cars IS electronic NOT mechanical).. could be a bad sensor reading..
Its connected twards the back of the transmission, on the drivers side. You'll see a couple of wires and a small plug looking thing..
It could also be that the spedometer plate (thats what I call it) lost connection with a few pins behind it. If you have ever taken the dash of your stang apart.. you'll see that the gauge faces are actually devided into 3 sections. The water temp and Fuel level are one of them. The spedometer, odometer and trip-meter are another.. and finally you have the oil pressure and voltage gauges on the last.
Behind them is a circuit board that they connect to using a few small pins that pertrude from behind the gauges faces.
You dont have to worry about being SUPER carefull with them.. the pins are fairly robust. Just dont take a hammer to them.. heh
Anyway, there's some ideas for ya. Hope this helps.
By they way, to get to the gauges, you have to remove the headlight switch.. wich can be a pain if you dont know how to do it.
There is a small metal tab inside of the switch housing.. rotate the knob until you see the small opening in the side.. then lightly pry the metal tab outward.. the knob should then pop right off.
good luck dude!!