Rear Drive Ratios,
BMW Airhead Motorcycles

Variables for the chart, below, are your actual on-the-road tire diameters (modest effect); tire pressures/temperature (tiny effect). The variation in UNLOADED diameter of a 4.00-18 rear tire versus a 120-90×18 rear tire is about 15 mm in the WORST case I know of. However, the actual rolling circumference varies little (probably about 2%, but could be larger). Hence the values shown below are THEORETICALLY fairly accurate, and some are taken from a BMW chart dated 1978, others are calculated, and some are actual test data.  Values are theoretical; and do NOT account for tire slippage and tire variations, nor for speedometer and tachometer variations.  Because of these factors, rpm is LIKELY higher for a true speed as charted.

See notes at end of this article!!  The speedometer ratio is printed on the dial of most speedometers.  The author’s website discusses things in much more depth, and includes expanded ratios for earlier models, etc…..link at the end of this article.

NOTE 1. Above are for original stock size tires at recommended original inflation pressures. Values are theoretical; and do not account for tire slippage and tire variations, nor for speedometer and tachometer variations.   Values are for statute miles per hour.   Information on Kph figures for the speedometer ratios, etc., are in the author’s website version, link is at end of this article.

NOTE 2. If you have a standard 5 speed transmission, the following is in 4th gear:

NOTE 3. Speedometer ratio is usually printed in small characters on face of the speedometer. The authors website article explains the figures in more depth.  BMW speedometers are notorious for reading HIGH, and this was done ON PURPOSE, and the authors site explains why and how much. The 85 mph speedometers are often quite accurate.

NOTE 4. It is UNlikely that you will have the same exact results as in the table, due to slippage, accumulated errors, tolerances, etc. It is unlikely that your RPM for any given road speed will be the same as the table indicates. Probably the rpm it takes for the table speeds will be about 5-8% more, assuming little slippage at the tire, and that other accumulated errors are minimal. This is about the amount many stock speedometers are in error.  However, BMW, on purpose, does not usually have 100% accurate speedometers (but the odometers ARE accurate).  Information on how BMW calibrates speedometers is in the author’s website.
Examples: If you have a 3.20 rear gearset, expect closer to 62 mph at 4000; 3.44 expect 57 mph at 4000. In actual on-road measurements, you can expect as much as a 15% higher rpm, for a given TRUE road speed….if the accumulated errors are additive, which they are usually NOT (some add, some subtract).

Note 5. Prior to the /5 series, BMW had very different sets of rear drive gear ratios. See website, URL is at the bottom of this article.

Note 6. The most accurate methods of calibrating a speedometer are, in NO specific order of accuracy:

• Mile posts and stopwatch
• Radar gun
• GPS (this is the easiest, for most riders)

Calibration is best done on a clean, dry, and straight paved road that is rather smooth, as surface irregularities will cause tire slippage, and you most often do NOT feel slippage until rather extreme.

Note 7. Measure the radius of the tire, bike sitting on tires (not stands), you and any passenger seated on bike, bike pointed straight ahead, and balanced straight up, buddy measuring the DEAD CENTER of axle to the floor, multiplied by 2, multiplied by pi.  This will give the working circumference close to the actual riding 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 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: Early BMW Airheads came with a 4.00 x 18 rear tire. That tire is likely to measure about 80 inches in circumference. The formula will show that for a 70 mph speed, the tach should be reading 4667 rpm.

Copyright 2021, R. Fleischer

Vastly expanded information is available on the Author’s website:
https://bmwmotorcycletech.info/ringgears.htm

Snowbum #1843
https://bmwmotorcycletech.info/index.html