Hey there everyone. I am looking for someone that knows what the ecm's looking for in an 870?

A quick rundown of what I have done:

Jacked the 85mph cable driven cluster out and put in a 140mph electronic unit. Used Painless Wirings VSS sending unit part#60015 to plug into the ECM and Speedo.

The 60015 is a 2000ppm unit for 700R-4s. From what I have read, it SOUNDS like the stock unit is 4000ppm, and then is plugged into a buffer box which drags it to 2000ppm for the cruise control and ecm...but the speedo gets a "pure" signal at 4000ppm. Is this true? Did GM really have their heads up their asses? Why would they put a 4000ppm speedo in when everything runs at 2000?

Reason I am asking is, I took it out for a test run. Got no VSS codes, but am only running at about half on the speedo...(at 40mph it shows 20). This is "ok" for the time being as this is a daily driver...but is the ECM looking for 2000 or 4000? I don't want to mess up the PE tables, fan kick on,or TCC. Any help? Thanks!!!!