Yea, the original speed sensor in a 280zx was used for activating the cruise control only above 35 mph, and for knowing when the car was moving to warn you about the parking brake, door ajar, etc.
I believe the stock ECU uses air flow, air temp, coolant temp, and RPM. I belive that it has 3 predefined values, off idle, cruise with minor variation through the O2 sensor, and WOT.
The ECU uses the TPS to know when the TB is closed, and when it is opened to add off-idle enrichment. It goes into WOT mode under high load and anything above 3500 rpms.
The Turbo auto ECU goes in WOT by seeing high load AND the kick down switch (which is why some people who use the auto ECU in a 5 speed swap go lean under WOT if they don't leave this connected).
Mario