It is a speed sensor. Some of the High-altitude or California models came with them to activate some sort of soleniod for emissions purposes.

By cody_guldstrand@hotmail.com (Turbo74) (4.46.69.173) on Monday, December 20, 2004 - 08:31 pm:

long story short, i put a deluxe spedo in my car and when i was putting it in i noiticed it had some type of speed sensor. it would start reading when it would come off zero and it would stop when it went past 100mph. i did not have any thing to plug it into so i unplug the sensor from the gauge. does any one know what this is??