Is it not possible to use the factory sensor? As it's already in the gearbox, and it is what the factory auto computer uses.

I have this setup on my desk at the moment all connected, and by running a piece of metal past the sensor I can read the pulses with the pic. Looking at the pulses on the oscilloscope gives me the indication that it would be ok to use.

The other issue about using optical sensors is that this is on a 4x4 which will see a lot of mud and water and the like.