When I had the retrofit done in my E46, I had exactly the same issue - in a nutshell, it transpired that the retrofit loom as supplied by BMW had a wiring error whereby the wheel speed sensor pin the the connector that attaches to the Nav Computer wasn't in the correct 'hole' in the connector shell. IIRC it should have been in pin 11 but was in pin 10 or perhaps the other way round.

Check the WDS for the correct pinout on the Nac Computer for the Wheel Speed sensor (or do a search here as I'm sure it's been posted before) and then cross-check with the wiring you have in your loom.

Once it was swapped over on mine, the accuracy to turns/junctions etc worked perfectly.

HTH.

Edit: Just found the original article re. this issue:

http://www.xoutpost.com/articles/all...formation.html