Electronic Speedometer. How does it work?

[rscam]

I have put an NV3500 in my truck and it has an electronic speedometer. I have seen numerous posts about conversion units. From what I've seen you can only buy a converter to go from Mech. to Elec. Since the output on the trans. would be electronic. I would have to switch to an electronic speedo. My ? is if I get an Electronic speedo. out of an newer car do I have to have the ECM from the car or does the transmission speed sensor send its signal directly to the speedo? If I am correct the earlier cars and trucks say early 90's that were electronic speedo. The ECM was only used for emissions and fuel delivery. I imagine the newer cars have the ECM involved in everything.