i also know the spd signal is used for auto trans shifting. the ect ecu looks at sp1 (speedo signal) and sp2 (rear speed sensor). if the 2 signals do not match it will set a code 61 and will go to a default shift table. also, it is a digital signal, you can see it with an o'scope. i can't confirm if it uses the spd signal for fuel mapping, but the above i do know for sure.
marlin rock crawler makes an adapter for the speedo, and dakota digital sells a signal box with analog/digital outputs.