How do I calibrate my GPS. For some reason on some roads it thinks I am 50 to 100 feet off the side of the road. Then some roads I am fine. It is real annoying when the navigation lady keeps telling me to turn left at the intersection of 1st and broad, 2nd and broad, etc. for every side street I pass because it thinks I am 100 feet to the left of where I actually am. I have never had this problem with any GPS I have ever used so what gives..