Just completed a weekend run between IL and NY. I took a stop watch, and did both Speedo and Odo checks. Also repeated in 2 states, just in case the mile markers are different.
First I set the cruise on indicated 75, and with no interruptions, covered 20 miles in 17:02. That calculates to just over 70 actual mph. I also checked the Odo, and found that over 45 miles, the total error was only 0.3 miles fast.
My question is this; If BMW can electronically get the Odo correct(0.66%), why is the Speedo so far off(6.5%)??, unless the electronics are purposely calibrated to give such a great error.
For those into top speed runs, assuming linear error, the top speed limited at 128 would indicate 136 on the speedo.
Anyone with similar results??

That is typical. The typical answer is liability. BMW would be in a very bad position is the speedo said 30 when you were actually going 35 and caused an accident where there was severe injury and/or death.