My GPS is normally reading 2-3 MPH below my speedometer. It depends, on the car, most of the speedo's are off a little, but it shouldn't affect the odometer. If you ever upgrade rims sized or something of that sort, that could cause it be off. The dealer might be able to help you calibrate it. I just go with the flow, and avoid being the fastest on the road (unless no one is around) and the exact speed doesn't really matter.
