Variables for the chart 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 120-90x18 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.