[Walt Fricke]

Clean up the battery to chassis ground first. Most important for instruments, and easiest to reach. Get a voltmeter that plugs into the lighter socket to check voltage with high loads (like at night).

The sensor and its wiring isn't part of the problem. But the sensor ground is from the speedo ground, so if speedo ground has some resistance, could influence things. Signal is just a square wave from a reed switch opening and closing 8 times per wheel rotation. If that charges/discharges a capacitor in the speedo, I can see low voltage could do this.