Me too! I just discovered this issue today in the 2014 3500 Laramie Megacab 6.4L Hemi 4.10 with 98k that I've owned for just a few days -- except the difference in my speedometers is even bigger. I had been relying upon the dial, but just realized it's reading about 10% low as one of those "Your Speed" signs telling me I was speeding prompted me to switch the MPG display I was running to the digital speedometer.
The digital speedometer matched the "Your Speed" sign; my dial is reading at least 10% low (so I'm really traveling at 34mph when I'm holding the peg at 30mph).
The truck was manufactured for Canada then after a year immigrated and was naturalized and went through the U.S. conversion process. I'm guessing the thing has been wrong since the instrument cluster got changed but the previous United States owner wasn't bothered enough to change it.
If it's not too much, I'd like to get it dialed-in, but I don't want to screw up the digital one that reads correct (and which I assume is associated with the digital odometer).
What's the fix?