These guys took a road trip to the Dauer headquarters in Nurnberg, Germany and met with J. Dauer himself... interesting site with pictures and info on the Dauer 962.
http://www.bmwm5.com/articles/german...trip2001-3.htm
In short: Of the original 50 planned just 11 were completed (3 of those currently rot in the SoB parking lots as all of you who follow Sultan matters know...). The 404.6 kph/251.46 mph top speed was apparently reached in the VAG Group oval track near Wolfsburg, Germany so it's not just a wild claim or estimate (there's a video to download too but I haven't - connection too slow). If they used street tyres for the run (and I guess they did since they claim the car to be TUV compliant) this record stands as far as I'm concerned (and as it was a car you could buy and not a prototype I guess it qualifies as production)