Speedometer jumping around

[Foehammer]

Hey guys, haven't been on here for a while. My question is today while sitting at a stop light my truck's speedometer and tachometer were hopping around, the speedometer more than the tach, the speedometer was hopping around at about the 15 mph mark while not moving and the idle seemed a little rough, but not horrible. I drove it home with no issues but let it run a few mins at idle while parked in my driveway and the speedometer skipped around again for about 15-30 secs and then went back to normal. What could be causing this?

 

[Foehammer]

Quick update: I just took a drive to the store and the speedometer and tach jump around only while in drive not in neutral or park, it seems to be worse when I press on the accelerator after turning a corner or after breaking or stopping and getting back up to speed, while cruising it seems to level out and not skip around, but when driving in my neighborhood at around 20 mph the speedometer jumps around near 30 mph and the tach moves around erratically while the engine is running smoothly. Thanks in advance for any suggestions guys.

 

[Raleigh]

My speedometer is doing something similar.
It started by bouncing around like yours but then it started jumping to 85 and down to 0.
Now it just doesn't work at all!
I never had any issues with my tach, the tach signal comes from the alternator FYI.

I am pretty sure that the sensor is to blame, but I haven't wanted to spend the $70 to find out so I just deal with it.

 

[3500GMC]

Grounds or maybe an alternator. Not sure but there might be a list of grounds to check in the 6.5 tech ref section on this site.

I know that grounds have been a problem for guys in the past. I've been lucky with the '93, it's relatively simple electrically.

 

[JMJNet]

Not sure on the speedometer but for the tach, it is the alternator going out as 3500GMC said.

May be it is just a simple ground issue also.