pmckechnie - I do not see a ground wire from the speedometer unless it is the one plug with a tab at the bottom which plugs over one pin in the center speedometer. You can just see it in this pic as a black plug resting on the wood of the dash; I had just unplugged it from the instrument cluster. Is this what you are calling a ground wire?

On the driver's side I found three connections - this black plug which slides over a single pole on what I believe is the center of the speedometer; guessing this makes the needle turn on the speedometer. A multi-plug which is rectangular and has eight pins. And a round multi-plug with about a dozen pins and a somewhat triangular plug in the center which helps you figure out how to guide it in.

So I understand that the copper? tab or bar on the bottom of the speedo is probably a ground and maybe touches the tach somehow. But I cannot figure out your comment about a second ground wire unless it comes from the center of the speedo.