BMW odometers tend to be quite accurate, and all the speedometers EXCEPT the 85 mph ones (which are usually very accurate), tend to read high by 6-12%. A chart comparing speedometer face ratios, road speeds, rpm, gear in use, ETC., is on this site at "Final Drive Ratios"; and a more detailed on is on the Author's website.   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 proper rear drive ratio, see the charts on the author's website in particular, which is more expanded than on this .org website.

 http://bmwmotorcycletech.info/

1. You can calibrate a speedometer by using the seconds sweep hand or digital seconds on your watch, versus posted mile markers. You can 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 calibrate the speedometer (and odometer) quite accurately.

3. You can use an electronic digital meter to calibrate your tachometer. I have seen meters as cheap as $35.00 that will measure engine speed....and dwell angle...volts, amps, etc., all in one instrument.

4. You can follow the Club's Canon #2, and do what we did decades ago; that is, make up a round piece of cardboard or plastic, and mount it to the alternator bolt or camshaft (on pre-1979 models), place marks on it, and for a stroboscopic light source use any AC line powered fluorescent light. It would be easy to get a stopped strobe effect at 1200, 1800, 2400...etc., rpm. One can do this with the rear wheel, off the ground, and spinning, with chalk marks...fun, huh?! THAT is one case where I advise you to ignore Canon #2, as it is dangerous.

5. You can send your speedometer or tach to a speedometer shop.

6. You can have a buddy with some law enforcement agency use a radar gun on you.

7. The speedometer ratio matches the rear drive ratio, see the chart. See the chart posting, for which speedometer matches which rear end ratio. Speedometer ratio is easily changed by a speedometer shop. NOTE that due to some of the speedometers already reading fast, if you change the rear drive ratio, it MIGHT now read closer (or much worse).

8. Measuring the radius of the tire, bike on tires, not stands, you and passenger seated on bike, bike pointed straight ahead, and balanced straight up, buddy measuring the CENTER of axle to the floor, multiplied by 2, multiplied by pi, will give the circumference close enough to the actual in-use number.

9. The formula for determining the relationships, suitably simplified is as follows:

Let T = the tach reading
Let M = miles per hour
Let C = circumference in INCHES
Let S = small number in the rear end ratio
Let L = large number in the rear end ratio

example: you have 37/11 gears (which is 3.36:1 btw). S = 11; and L = 37

THEN, multiply the following: (T)(C)(S)

Divide that result by (1584)(L)

The result is M

Rearrange this formula to find any of the values, like you learned in jr. high school in beginning algebra.

Practical example: Most early BMW's came with a 4.00 x 18 rear tire. That tire is likely, even if you have an oversize 120-90 x 18, to measure about 80 inches in circumference.

The formula will show that for a 70 mph speed, the tach should be reading 4667 rpm. NOTE that this is theoretical, and there is tire slippage and other effects.

Snowbum