NOTE 3. speedometer ratio is usually printed in small characters on face of the speedometer. BMW speedometers are notorious for reading HIGH. The 85 mph speedometers are often VERY accurate.

NOTE 4. It is UNlikely that you will ACTUALLY have the same results as the table, due to 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 no slippage at the tire, and other accumulated errors are minimal. Interestingly, this is about the amount many stock speedometers are in error. 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 my website, URL is at the bottom of this article, below.

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

• Mile posts and stopwatch
• Radar gun
• GPS
  

Calibration is best done on a clean, dry, paved road.

Note 7. OLD rear drives, without the vent, generally used SAE 40 oil. I do NOT think that straight weight oils should be used ANYplace on any airhead today. With the vent it is a must to use hypoid oil, 80W90, non-synthetic. Rumors (??) have it that hypoid oil in the ancient drives will destroy seals, or damage bronze/brass bushings, or?....totally unconfirmed by myself.

Note 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 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 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.

Snowbum #1843