Unfortunately my Englisch is not the best, but I will also try to answer you
You really should try to calibrate the system, take a look at this
thread , especially on page 3 the posting from "Mister B" with the pdf-file should help you
You can also use the search-button in the forum, try "navi-kalibrierung" or word-meanings like this and you will find a lot more information, too
regards
7er Fan