PS - is the sensor itself well away from the HT leads and ignition coil?If all else fails you could try a R-C filter on the input pin. Something like 1K between the pullup/hall sensor and and the pin, and 10nF or 100nF from the pin to signal ground. But this should not be necessary if your sensor uses shielded wire and is correctly grounded
What?! where do you want to mount the reed-switch? I am mounting it on my shock, and I have a magnet expoxy'ed into my break disc caliper... I suppose you guys all ride superbikes where that makes a difference... (me : XR650L, KTM950SE)
You do know the difference between a tachometer and a speedometer, right?