Yep, thats how mine works. Dead reckoning allows it to be more accurate than just using the GPS signal.

The signals come in 2 flavours according to my nav manual, either pulsed or a voltage which increases with speed. If you only have a mechanical speedo you can fit a transducer which gives 4 or 8 pulses per revolution.

But the big problem is the input to the empeg, According to Hugo, if you use both the mute line and the light sense, there are no free GPIOs left to use.

Maybe the new tuner will resolve this.

Then, of course, someone still needs to write the code to make use of the signal