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Ring & Pinion Gears & their Ratios. 
RPM and Road Speeds versus ratios. 
Instrument numeral colors, calibration, etc.
Airheads and slash 2

Copyright, 2014, R. Fleischer
Article #48

Website for how to open-up and do some repairs in your instrument:

Find the article, under Technical tips.

for the /5, see:


Speedometer & Tachometer parts and service work:   

Instrument housings, NEW:

Terry Vrla; 776 SE 3rd Ave; Hillsboro, OR 97123; 503 421 5782
Airhead instrument pod overhauls, repairs, parts,etc. Speedometers,
Tachometers, can even fix the threaded insert areas.  Highly recommended!

For ODOMETER gears, all models:  Repairs and calibration of
speedometers, odometers, mechanical & including the electronics type on the K
bikes. I think they can do actual circuitry repairs.  Rick Borth

North Hollywood Speedometer

Palo Alto Speedo in California.

West L.A. Automotive
877-900-9009 310-838-7350 In Los Angeles

Southern Electronics, Inc, in Richmond, Virginia do speedo/tach repairs....and can
do things quite in-depth. They can do mechanical and electronics units, even BMW
radios. 800-446-2880; 800-572-2880
Their radio site is

One specific item!....whilst well known for his driveshaft repairing, he carries
the small rubbery button that fits over the button on your odometer on later
models...a hard to find goody. Guy Hendersen (209) 962-7500 Hendersen Precision; 22105 Feretti Road,
Groveland, California 95321

NOTE 1.    Variables:
Your actual on-the-road tire diameter varies from that which is in manufacturer's books, or that which is calculated. Most ROAD type tires have a rather close tolerance as to diameter...BUT, the tire is NOT round where it contacts the pavement.  There is also a modest effect when the tire is new to when well worn.  Even in same size, tires vary a small amount by manufacturer and model, sometimes considerably though (this is usually with off-road tires, like enduro tires, which can be widely variable in the same size as a road tire...especially width).   Tire pressures/temperature have an effect, so does the rim width; but those are usually very small effects.    Larger effect:  SLIPPAGE of tire rubber at the road surface. You do NOT have to, and usually do not, feel the slippage, it is always occurring, except when the motorcycle is not moving.  Because it is the rear tire that drives the motorcycle, if the speedometer and odometer are driven by either wheel, the result varies with WHICH wheel.  Manufacturer's compensate for that, of course.

The unloaded to loaded measured diameter of a road-type 4.00-18 rear tire versus a 120-90x18, typically the largest rear tire on early Airheads that is ever used, is only about 15 mm in the WORST case I know of.  The actual rolling circumference USUALLY differs by only about 2%.  Hence the values shown below in the text and charts are THEORETICALLY reasonably accurate, and many are taken from a BMW chart dated 1978, others are calculated, and some are actual test data.  Keep in mind the variables in the paragraphs above!  Be sure to read the various notes in this article, regarding such as BMW's WRONG conversion of Kph to Mph, regarding the ratios that the speedometer/odometer use.

              The values in the chart, below, are THEORETICAL rpm & speeds for original stock size tires at recommended
              original inflation pressures, loads, ETC.  While Mph figures are shown, I suggest you use the Kph speeds, and
              YOU convert from Kph to Mph, & disregard BMW's Mph figures, as those are shown here with BMW's own original

It is UNLIKELY that you will ACTUALLY have exactly the same results for rpm (and speed) as in the table, due to
              accumulated errors, tolerances, etc.  It is MY BELIEF that BMW did NOT chart these things for ON THE ROAD; but
              under some factory controlled setup or calculations, and it is MY BELIEF that you will, assuming accurate instruments
              in and of themselves, ALWAYS have a higher rpm ON THE ROAD, for a given speed.  Probably the rpm it takes for
              the charted speeds will be about 5-8% more.
Interestingly, this is about the amount many stock BMW
              speedometers are in error...ON PURPOSE!  see NOTE 3.  AND, do not forget what I said about YOU converting
              Kph to Mph (and NOT vice-versa!!!).

NOTE 2. If you have a standard 5 speed transmission, the following is in 4th gear:
                2.91  32/11  4320 rpm for 70mph; 7200 rpm for 117mph
                3.36  37/11  7200 rpm for 101.3mph

NOTE 3.  BMW speedometers are notorious for reading HIGH.  But the 0-85 mph speedometers are often very accurate.
 The ODOMETERS on ALL are usually quite accurate. When a rear drive RATIO is numerically LOWERED by ONE
               step, the speedometer error, if present, is OFTEN eliminated, without any need for speedometer internal work or a
               new speedometer.  The ODOMETER will be off, however. 
               BMW, in general, purposely calibrates its speedometers to never read slower than the actual speed the
               motorcycle is traveling at; taking into account tire variations, temperature, instrument changes with
               conditions, and so on.  BMW has a bulletin on this, #2756, dated 10-21-1996.    The bulletin states that the
               MAXIMUM amount the speedometer will read fast is:  10% of true speed, PLUS 2.4 mph.

NOTE 4. The most accurate methods of calibrating a speedometer are, in NO specific order of accuracy:
                a. Mile posts and stopwatch
                b. Radar gun 
                c. GPS 

NOTE 5. If you are really anal:  
              Have someone measure the radius of the tire, bike on tires (not side-stand nor center-stand), you and passenger
              seated on bike, bike pointed straight ahead, and balanced straight up.  The bike MUST be square to the surface!  
              Your buddy measures the CENTER of rear axle to the floor.  If you have no square or level to use, have your buddy
              measure the center of the rear axle at both its ends, and YOU adjust the lean of the bike until both measurements are
              identical.  Take that measured true distance from center of axle to floor; multiply it by 2; then multiply that by pi (or by
              22/7)....and that will give the working circumference close enough to the actual in-use number. 

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). 
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.  Compare THAT to the chart (4486 rpm).  You can see that this goes right along with what you already know...your speedometer reads faster than you are going. ..........AND, the tire slips, causing more errors.

Suppose you want to know the engine rpm in any particular gear, for any particular tire and road speed.   Transmission ratios are in your owners book or on this website.  Rear end ratios are stamped into the rear drive.   You need to have a helper very accurately measure the radius of the tire with you and normal load, bike balanced on its tires as previously noted.     Select a mph speed, multiply that by the overall gear ratio which is the gear ratio of the rear drive and the transmission gear ratio (multiply them).  Multiply the result by 336 and divide by the tire DIAMETER in inches.   The result is the engine rpm.

NOTE 6:  The single sided (Monoshock) and the Paralever models have a limited number of gear ratios available.    3.36 was popular on the ST and G/S (R80RT also) & late R65; and 3.00 was on the late R100RS and RT; and for the R80GS, R80R, R100GS, and R100R, you will likely find only the 3.20 and the 3.09.

NOTE 7:   BMW used different colored dials and pointers depending on model/year.  BMW used different plastic covers for the speedometer-
                tachometer and lights pods:
                1973-1977 had the indicator lights at the bottom.
                1978-1981 had the brake warning light at the bottom.  1978 was the transition year for the electronic tachometer; and BMW
                    changed the numbers and text from white to green.
                1981+ had a blue light plastic lense for the high beam indicator, and it was at the bottom.
                 Speedometers were 140 mph (with kph in smaller text); or, 220kph for metric countries.  There were 85 mph speedometers for
                     the USA during the 55 mph speed limit years.


The following chart is for the Airheads, which means  /5 and later, through the end of Airhead production in 1995.  Another table, below this first one, covers earlier models.

Figures are the PUBLISHED figures & are NOT correct with regards to MPH/KPH if you do the conversion.  BMW wrongly converted Kph to MPH in its bulletin.  BMW's original chart used 101 Kph to be 55 mph.  The correct speed is 62.6 mph for that conversion.  If you convert backwards, from Kph, to Mph, you may do better.
As always, assume correct speedometer ratio shown (and in reality, actual), and correct rear drive ratio, you can do a comparison with a GPS on a flat straight road; and, you can use a precision tachometer to check your tachometer.
Various checks can be done in one's garage, using photo tach and photo pulse counting equipment, etc.

Regarding the speedometer "ratio" (the tiny numbers located on the speedometer face, except most /5 models), if you intend to use these numbers during testing, such as with a known rpm source driving the speedometer directly: 
        The applied rpm is multiplied by 1000, and is for Mph (or Kph as case may be).  Thus, a ratio of 1.112 really
         means 1112 turns of the speedometer cable PER MILE.  NOTE that it might be expressed as W=1.112 or
         W=1112.   W=0.737 means the cable turns 737 times PER KILOMETER....example of the use of that information, might
         be that if you rotated the cable at 737 rpm, the speedometer should indicate 60 km/hr (60 kph), or 1 per minute.
Speedometer gear ratios can be shown in literature in two ways.  An example would be W=760.  The other way to show
         that is W= 0.760.

Instruments may not conform to precisely the W figures shown.  In motorcycle shipments to some countries, requirements for more precise accuracy (read that as USUALLY less optimism on speed) means that a slightly different W figure is used on the instrument.   It is ALSO my belief that BMW made some speedometer ratio changes even for the same ring gear ratios now and then that were not necessarily for Country regulations reasons, and this may be ANOTHER reason why some books show different speedometer ratios.   I suspect BMW did this just to make minor corrections in instrument readings.

NOTE:  The ratio between speedometer ratios, Mph/Kph, is ~0.62, always.


Ring Gear


55 mph

70 mph

7200      rpm




 on rear drive ratio; speedometer ratio, colors of numerals on the speedometer face




Racing ratio used on /5 and later.  See last note in this right side column for others.








White numerals 1974-1977; R100RS, R100S.   1981+ all had green numerals.  Some R75/5 (to frame 2973204) had 2.91 and not 3.20.  Some R75/6. Kph ratio is .665 & mph ratio in some literature as 1.0625.  

R75/5 with 200kph speedo: W=660
With 120 mph speedo, 120mph W=1050








White numerals 1974-1977; R90S, R100/7. 1978-1980, green numerals on some R100/7.   1981+ all had green numerals.  White pointer.  The R100; R100T; R100RS; R100RT, some countries and years, had 3.00.  In general, from 1987, all USA/UK bikes had 3.00 ratio on the monoshock models RS/RT.








White numerals 1974-1977; R90/6.   For 1978-1980, was green numerals, used on some R100/7.   3.09 was also used on R100R and R100GS and SOME R90S








White numerals 1974-1977; R75/5, R75/6, R75/7.  For 1978-1980, was green numerals, used on R80/7.   3.20 was also used on R80R, some R80GS

R75/5 with 200 kph speedos
W= .715.
With 120 mph speedos  W= 1155








White numerals 1974-1977; R60/6, R60/7.   Some old books have R60/5 & R60/6 as 3.36:1 rear drive & 0.766 as Km ratio, mph ratio as 1.266.  3.36 was also used on some R80G/S, R80RT, R80ST, and R65 from 1986.

R60/5 with 200kph speedo W=760.
With 120mph speedo,           W=1215.







0.793 ?

R65.   Some had 3.56.
Some manuals say the speedometer ratio is 1.267 for the 3.44 gearset for miles (.793 for Kph)








White numerals 1974-1977.   R65 models had 3.56 & 3.44.  R50/5 and R60/7 had 3.56. Books may show Kph ratio as 0.811, miles ratio as 1.297.
With 200kph speedos, W=800.  With 120mph speedos will have W=1300.






R45S, 35 hp model (at 7250 rpm)








The 1978 models came to the USA with green numerals and lines.  Earlier models had white numerals and lines.  Note also that the 1978 had an electric tachometer; with some changes in the instrument pod terminal wiring.



Not shown are some few ratios used by the factory and NEVER sold, even for racing; a FEW WERE used by the racing department.   For the 2.812 (31/11 gears) and 2.75 (33/12) ratios: while the books and on-line fiche show the 31/11 being used on numerous models, I have never seen it; it was for racing anyway.  The  2.615 (34/13) was available for the R75/5 racing.
There was a 4.50 ratio (36/8) for competition.


The BMW /2 series had a very different set of rear drive gear ratios. Some were specifically for sidecar use.  HERE, /2 means earlier than the /5 models, even though these may not be true /2.

25/8     3.13

 This was standard on the R69S.

35/11   3.18

 This was standard on the R50/2.

27/8     3.375

 Also used on the R50/2

32/9     3.556

25/7     3.58

 Used on the R50S in 1960 to 1962

27/7     3.86

 Sidecar use, 1956 to 1967

35/9     3.89

25/6     4.17 


26/6     4.33

 Sidecar use 1960-1969

35/8     4.38

27/6     4.50  R25/2

32/7     4.57 

36/7     5.14

 R25/2 with sidecar

26/5     5.20

 Optional ratio for R27 

Here is an on-line calculator for various bike tires.  You may find it useful:


01/27/2008:  Incorporate all previous revisions.  Rename article to avoid problems with publishing its URL to E-mail messages.  Modify article so the data is presented in TABLE format, so that it is vastly easier to read, no matter what browser is selected, nor screen size.
06/14/2009:  slight cleanup
04/27/2010:  fix inadvertent typo that had 60 mph in the sentence on speedometer cable turns per mile.
04/18/2011:  add more data on 3.89 and 4.25 ratios
06/10/2011:  minor revisions. Add hyperlink to .org website article.
11/12/2011:  Checked a few areas for accuracy
07/12/2012:  updated R65/R45 information very slightly
10/13/2012:  Add QR code; add language button, update Google Ad-Sense code, minor other coding
08/20/2013:  Add note about 1978 colors
10/06/2013:  Add UNconfirmed values for /5 bikes
11/20/2013:  Add tacomaworld hyperlink
02/05/2014:  Add info on 2.75, 33/12
08/06/2014:  Clear up some confusion on Euro speedometer ratios
12/18/2014:  Explain why the confusion.
06/11/2015:  Clarity, 33/11, and add note regarding 0.62 ratio between Mph and Kph.
10/15/2015:  Clean up article.  Some further cleanup and added notes on 11/04/2015; re-arrange some of the article.

Copyright, 2014, R. Fleischer

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