have been driving with it for over month with it gps still behing the car for about 30m.

are you sure, that what your screen displays, deviates among 30m from reality? in my case, the difference is about 5 to 10m and it recalibrates itself, when i turn left or right on a crossroad.

you wrote, that you upgradet from MKII to MKIV. perhaps the gps-reciever is "too old" and it recieves only a fixed amount of signals.
the accuracy depends on your environment, so it should be less, when you're riding downtown, because of the skyscrapers, or through a tunnel.

There is technical document over this in WDS, an if I remember correctly it suggests to make circles (in a square or round about) in order to adjust your GPS. Try small 3-4 circles in the same spot, then in opposite direction.
I hope this would fix your problem.

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Are you getting 'Wheel Speed' indications in the diagnostic screen? If not, then this will show your position as being out when compared to reality.

Had this issue in my E46 where I was always ahead of the map pointer by 20 oe 30 meters - the dealer had used the correct full retrofit kit when they installed my nav, but some of the looms were faulty from the factory and had the wheel speed sensor wire in the wrong connector. As soon as it was put into the correct socket, the accuracy was spot on. This speed signal, plus the input from the accelerometers is used to 'correct' any accuracy issues with the GPS and to keep the map updating correctly when you are out of GPS coverage such as in a tunnel.

It may be that there is a fault with the speed sensor line, or the loom is bad/faulty/miswired.