Are you honestly thinking about going to court and telling the judge that your car is physically incapable of going under 30 MPH? In my opinion, just go to your scheduled court date, dressed respectably. Unless the cop is a total asshole, he/she will probably meet with you before you're called up and change the speeding charge to a lesser infraction. Don't forget to call the judge "sir" or "ma'am". I tried this and it worked - speeding became failure to come to a complete stop at a stop sign. I still got points on my license, but my insurance rates stayed the same. Good luck.
BMW - With great power comes great responsibility.