Tachometer, Speedometer, and Odometer Calibration;
How to check the calibration/accuracy
BMW odometers tend to be quite accurate, and all the speedometers EXCEPT the 85 mph ones (which are usually accurate), tend to read high by 6-12%. This is ON PURPOSE by BMW. See the article hyperlinked just below, which is more detailed and has the information on why the inaccuracy, and MUCH more.
Recalibrating a speedometer is a touchy job, of messing with such as hairsprings, and is best left to an expert.
Speedometers have drive ratios printed on the face, and they must correspond to the rear drive ratio, see the charts on the author's website, which is more expanded than on this airheads.org website. The following hyperlink to the author's website has all the information you would want.
1. You can calibration-check a speedometer by using the seconds sweep hand or digital seconds on your watch, versus posted mile markers. You can also calibrate the odometer with a watch if you hold a constant speed, such as the very convenient 60 mph.
2. You can use a GPS receiver, on a STRAIGHT road, and with a good strong signal, easily check the calibration of both the speedometer and odometer quite accurately.
3. You can use an electronic digital meter to calibrate