BMW odometers tend to be quite accurate, BUT all the speedometers EXCEPT the 85 mph ones (which are usually accurate), tend to read high by 6 to 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, and is best left to an expert.

Speedometers/odometers have W 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 likely want at this time (there is also information on the site about working on these instruments):

https://bmwmotorcycletech.info/ringgears.htm

1. You can calibration-check a speedometer by using the seconds sweep hand or digital seconds on your watch or smart phone, versus posted mile markers. You can also calibrate the odometer with a watch if you hold a constant speed, such as a very convenient-to-use 60 mph.

2. You can use a GPS receiver, on a reasonably straight road, and easily check the calibration of both the speedometer and odometer.

3. You can use an electronic digital meter to calibrate your tachometer. I have seen such meters as cheap as $35.00 that will measure engine speed….and dwell angle…volts, amps, etc., all in one instrument.   There are also PHOTO-TACHOMETERS which use a tiny piece of white paper stuck to such as the alternator rotor (or, just chalk mark the end), and they read reflected light impulses.    Try such as Harbor Freight Company.

4. You can follow the Club’s Canon #2, which states “Airheads believe that the simplest engineering solutions are best”, and do what we did decades ago; that is, make up a round piece of paper or cardboard or plastic, and mount it to the alternator bolt. Place marks on it, and for a stroboscopic light source use any old-fashioned type of an AC line powered fluorescent light. It would be easy to get a stopped strobe effect at 1200, 1800, 2400…etc., rpm.  That’s for 60Hz power lines.    Some modern fluorescent lamps, including many coily types, do not get internally triggered by the power line frequency rate… you will need to check this.

5. You can send your speedometer or tachometer to a speedometer shop, such as Terry Vrla’s.

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

7. The speedometer ratio matches the rear drive ratio, see the chart. Speedometer ratio is easily changed by a speedometer shop. NOTE that due to many speedometers already reading fast, if you change the rear drive ratio to one step LOWER numerical ratio, it MIGHT now read closer.  You can figure this out by dividing the two ratios, and see if the % is near the % you need to change the speedometer ratio.   Be advised that changing the rear drive means the ODOMETER no longer reads correctly.

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 tachometer 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 a ratio of 3.36:1). 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, which 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 tachometer should be reading 4667 rpm. NOTE that this is theoretical, and there will be some tire slippage and other effects.

https://bmwmotorcycletech.info/ringgears.htm

Originally published in 2004, this article has been revised, if slightly, more than once, the version you are looking at is dated February 23rd, 2021.

Snowbum

https://bmwmotorcycletech.info/

Copyright 2021, R. Fleischer