What is the thing behind the speedo??

[mr48chev]

Originally Posted by craig32

Is that so it would turn the speedo at a different rate with a different axle gear? Kinda like changing the speedo gear in the transmission, only on the fly?

It was so that it would correct the speedometer to match either high or low range that the rear axle was in. When you flipped the lever/button to change ranges in the rear axle the valve also switched that box on the back of the speedometer to correct the readings.