Understandable, but it’s not the actual “speed” of vehicle travel - that is spot on. It’s the set speed display of autopilot, the one you change by scrolling up and down with the right wheel. It always indicates a set speed 1 mph lower than the MCU.It seems most of those add on devices lag by about 1 mph or so, almost no matter the make.
Or did you mean other aftermarket secondary displays show the autopilot speed setting incorrectly?