My tachometer barely moves without an input from me like stomping on the gas or the brake. Similar for the speedometer.
Are you driving at a speed where the transmission is hunting for a gear?
Couple of things to see if it affects the movement:
1. Try same drive in TOW/HAUL mode and see if that reduces the flutter.
2. Repeat same drive in manual mode, and keep the transmission limited to no higher than 3rd or 4th gear.
If some of these things reduce the flutter, then sounds like you are hunting for a gear to much. If no impact to the fluctuations, then probably is some sort of electrical issue. SavageBob probably understands the operation of the speedometer and tachometer better than most, definitely better than I. He may have some other thoughts.
When I watch my two meters, it appears that there is some sort of dampening built into the electrical circuitry. In old days when the transistor was a new fangled device, I would have suggested that my truck has a capacitor in the circuit for dampening purposes.
When you look at the speedometer, does the digital speedometer fluctuate as much as the analog device?
2015 Canyon SLT, Copper Red, 2WD, CC, SB, V6, Tow, blk leather, factory spray liner, safety pkg, generic roll up bed cover.