First off I want to thank DCB and all involved in designing the MpGuino! Sorry for the delay but I just now finished testing my new circuit to work with the inductive sensor and it works great at all speeds down to 3 mph now and outputs a pretty clean square wave. I could go lower but I was worried about spurious noise giving false triggers. I am using one channel of an lm339 quad comparator from Radio Shack configured as a zero crossing detector. I was having some annoying problems at first but it turned out my breadboard had a problem so the chip wasn't getting ground
One thing I am not sure of is whether to use a pcb mounted potentiometer on it to adjust the input threshold or just use a fixed resister. By adjusting the pot I think it would work fine even when using a 5 volt hall sensor. I'll go to Radio shack and pick up a small board to solder all the parts onto and take some pics. They should have everything needed to put it together. Thank you for your patience.