Motorcycle Tach Signal

Anyone have any idea what type of signal a Motorcycle ECM unit would be putting out for Tach and Speedo?

I have the wiring diagrams (2008 Harley Sportster) and the same 4 wires go into the Speedo that go into the optional ($500) Tach so I though, hey why not build on with the Arduino?

I assume that due to the wires being common between the tach and speedo that it is some sort of a digital signal which is being interpreted by the Tach and speedo but I am not really sure.

Any Ideas?

Speed could be estimated based upon rpm vs. transmission gearing. My car doesn’t even have a wire for the tach, it’s just a cable like a bicycle’s brake cable that spins… and attaches to the gauge cluster and goes to the transmission… and based upon how fast the transmission is spinning, gives the gauge in the car it’s mph reading.

Could be the j1850 data bus. If you can get the rpm and the speed from the bus, you can also determine the gear for a nifty gear indicator.