Im 99% sure that it is a different "protocol" something about pulses per second 400 to 800? I dont know exactly.
What I do know is mine reads 100% backwards. I floor it wot and I get 255 mpg I coast and I get 0. So I've been driving every where in 1st gear flat on the floor Bwahahahaha.
Good luck with this one. Hopefully more "experts will chime in.
CrewCab Steve
If I were to guess, I'd say you're thinking PPM, or Pulse Per Mile.
That's the output of the VSS. The PCM takes the PPM, interprets it (IE: what the DRAC/VSSB did in earlier trucks) and comes up with the road speed. It will also interpret the PPM and output different signals depending on what a component needs. IE: cruise may be looking for a different signal than the PCM calibration, Speedo is looking for a different output, the DIC may be looking for another, etc.
Seems to me if I were designing a system, I would have all the components reading the same PPM signal, but....well....that's GM for ya....