E39 Speedo Calibration - Pop Quiz

It seems the Euro standard for vehicle speedometers encourages BMW to err on the conservative (safe) side. When compared with my Street Pilot III, my 525i speedo is about 4-5 MPH fast at an indicated 80 MPH. It seems to be about 3-5% high across the entire range which is within the BMW spec. Question: can the analog speedo be calibrated to read correctly without invoking divine intervention or requiring a line of credit from my bank? I suspect there is a tweak somewhere to adjust the indicated output vs. known input. Help!
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