Re: Speedo's and gas engines...Help !!
If is is EMF, one could do what carmakers do for vital sensors that measure engine rpm very nearly exactly the same way our bicycle computers do... Twist the wire. About two twists per inch is more than enough. (Maybe even less, say one twist per 3/4") Then wrap it in a continuos spiral of foil along with a thin-gauge uninsulated wire and ground it to the frame. When the magnetic wave from the ignition hits the twisted pair wire, the voltage it induces into the wires will now be opposite each other, cancelling them out. Also, the sheilding will carry away some of it before reaches the wire. Note: the shielding isn't bulk related. You don't need more than a single thin layer (of course each spiral will overlap onto the previous one.) and it doesn't help any more to was it up around the wire, as the fashion police will stop you. Take a long strip, say, 8 feet or so, cut into a 2 inch strip. Lay it on the floor with the uninsulated wire taped every few spots to keep it from wandering, alongside your twisted wire computer pickup just a bit off of parallel. Roll the wire into the foil at a relatively shallow angle...say 30 degrees, overlapping each wrap about half the strip width, or an inch or so. After all wrapped, spiral on a layer of electrical tape to prevent foil tearing. Make sure it's grounded to frame using the uninsulated wire!
It's a lot of work, and twisting alone may be enough. If not, go for the foil.