[SkidmoreGarage]

That sounds like generally a good wiring strategy, I think the only thing to consider is whether you have multiple grounding ring terminals on the same lug. That can cause problems. Convention is to crimp multiple wires to the same ring terminal or to use separate lugs. But I wouldn't put money on that being the solution.

I made a big assumption that the speedo is a self-scaling analog circuit looking for a square pulse when I was thinking a ground/voltage offset problem. What does the voltage feed look like to the gauge/box/sensor look like? Does it share a circuit with anything?

I would also look at the ECU wiring, does the ECU have VSS in? Are you driving any kind of IACV? I'm not familiar with the sniper efi.

The signal at 30mph is I think around 1kHz, and should be linear. Have you called DD? It may just be a bad box.