Mechanical speedometers have a magnet spun by the speedometer cable sitting inside a steel bell-shaped dome. The faster the magnet spins the more the dome is "dragged" along by eddy currents induced by the magnet. The bell is on one end of a shaft and the needle on the other. In between there's a coil spring wrapped around the shaft that resists the turning force by the dome. When the car stops the cable stops spinning and the spring brings the needle back to the zero peg.
The Fiero speedometer uses what's called an "air core" meter movement. This consists of a needle on a shaft and a magnet on the other end of the shaft. The needle and shaft assembly is free-floating, there's no return spring. To move the needle there are two magnetic windings 90° apart that the magnet sits in. As you vary the current through the two windings the needle moves. There's a chip that drives the two windings, each getting a varying amount of current so that the combined magnetic fields move the needle's magnet to any given position. It's done this way rather than with a cable because a speedometer cable would have to be nearly 10' long to get from the transmission to the instrument cluster. An electronic speedometer has the added benefits of being more reliable, more accurate, and less costly to manufacture.
Edit to add: Because there's no return spring, the needles are free-floating and without power to the coils the needles wind up at random positions based on their weight distribution and friction on the needle shaft. In other words, it's supposed to work that way.
[This message has been edited by JazzMan (edited 05-20-2011).]
