I have 3 cages and only 1 is in sync with mt GPS as far as MPH. My Zumo 660 is a bit off from my bike also. Don't think you can trust the traffic radar to be accurate either.
I drive through one of those traffic radar speed signs too and it shows 53 mph when my Harley speedo shows 55 mph. When I go through there with my pickup it is spot on. So I guess the bike is off a little. New tires too, so that is not it.
My Ultra is off about 3 mph at 75 according to my Zumo. My old night train is dead on according to the gps. Only one of our cars is accurate. What I have heard is that they make them slow on purpose.
My speedometer reading upwards 4 to 5 MPH throws off the odometer. This product will pay for itself after awhile just with its accurate actual mileage contribution. Appreciate the referral and everyones feedbacks.