You have to use a GPS, especially those designed for vehicles, with a little common sense. I have a Garmin, and last summer had set Bend, OR as my destination. I decided to make a quick side excusion into some lava beds located in northern CA. It was amusing nearly every time I approached an old FR it would recalculate and direct me to turn north on these "roads". Some of the old FR's were barely a track in the brush.
This happens from time-to-time with everyone, I'm sure. In this instance the mis-direction was most extreme I'd observed. Given the scale of the number of maps the typical GPS can now load, it would be a near impossible feat to expect that every road appearing on a map would be actually passable.
