For the limited's you can see the difference on the nav map. Bring up mph on the screen and compare.
I'm not sure what you're saying here.
If you're saying that you can compare the normal speedometer gauge readout to the digital readout on the navigation display, that comparison isn't going to give you a useful result. Both of those readouts use the vss to calculate the speed, Harley doesn't display the GPS-calculated speed. You have to use a different instrument to determine the GPS calculated speed, such as a GPS (Garmin, Tom Tom) or a GPS app on a smart phone.
If the two displays don't match on your bike, it is likely that one (probably the speedometer) is malfunctioning. I have seen a few in the past with poor needle placement, or it could be failure of the speedo.