Keith
The VDO instructions say you need to drive for exactly a mile to auto calibrate, doesn't matter what speed it's the distance that matters. If there are a number of these markers over a mile I am in luck but I don't know where any are.
They are usually only a quarter mile. What you could do is measure the mile with your GPS that will be accurate. I suggest finding a quiet straightish piece of road and make your own markers, two white pieces or 2x1 should do it and measure using GPS.
