The speedo is merely a tachometer that registers rpm from a source relevant to vehicle speed - the ABS also uses those frequencies - what you're seeing is engine-related frequencies, part of the noise that resulted in the FSD filter requirement in the OBD1 trucks - the noise is still there on the busses, some instrument clusters are more sensitive than others, but all respond to the increasing frequencies from the CPS, OPS, FSD pulses, etc - the alternator is fixed-freq at all rpm, variable duty-cycle or pulse-width - input from VSS is zero when stopped, allowing speedo to sense any other frequencies - VSS output is full strength when vehicle is moving, so speedo ignores those other frequencies - my GMC does it - my Dodge does it - my Astro van does it - my Jeep does it - my son's FJ does it - no big deal as long as the odometer does not reflect the 'aliasing' mileage, eh.............................
BTW - if you duplicate a post, go back and delete one or the other - it's the courteous thing to do - keeps down forum clutter
