What causes the speedo to increase?

[Harqobispal]

When you have a 6.5l what causes the speedo to reach 10mph? The truck is in park, and you are just reeving the engine to 2k. I had this happen the other day when it was -2 out(Canandians and northern americans know the temp). I drove the truck around then it would not happen anymore.

 

[bk95td]

I think it may be the 6.5 gremlins in the cluster trying to drive you nuts.LOL Is it working? Mine will jump sometimes when the glow plugs cycle. I don't know why it does it either. Voltage spike maybe?
Why the double post? You can go back and edit.

 

[DennisG01]

Once in a while my speedo will jump around when I first start it. It only happens when it's really cold. Weird things happen in the cold... If it works fine otherwise, don't worry.

 

[gmctd]

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