Now that's an interesting failure. Of course the upside is the exhilarating sensation of speed while sitting at a traffic light, and the general improvement in the flow of traffic. Also, you can look forward to a sensational advantage in gas mileage.

Sorry, couldn't resist! I think Alan is right, but before swapping the speedo head you might want to check the printed circuit board (for the speedo itself, not the cluster) for contamination or cracked traces. That's if you want to preserve your odometer reading, otherwise you can pick up a speedometer on ebay for about $10.