The only thing I have against just putting the Dakota VSS signal into the EEC is the EEC usually has some logic that pulls a bit of timing above 100 MPH, double the signal and it will be falling flat at 50 MPH. You may never notice, or it may just show up as bad highway economy, or even feel a bit gutless on the highway.
Can the Dakota gauge work with other VSS? Almost every aftermarket electronic speedo I have ever seen has programibility to match existing VSS. If that is the case use the Ford VSS and share the signal with the Dakota gauge. I am 99% sure the Dakota gauge will be just fine and happy with the ford VSS.
What the EEC does with the VSS varys a little. The most notorous is prevent stalling on down grades. A long downhill grade engine braking without VSS and at the end the engine will like to stall as it was thinking it was running a high idle and didn't see the vehicle was pushing the engine, doesn't recover fast enough. Also part of the clutch switch logic. On applications with the E4OD trans, it gives the vehicle speed so the shift points are correct. Some logic for emissions controls (canister purge and EGR mostly), sometimes it can do a little fuel economy trick at light cruise and do a lean burn for gas mileage. I have seen some EEC programming that pulls a bit of timing at high speeds. Although not on the Mustang EECs, some could have top speed limiters as well. So that is what you could see the VSS controlling.
