wind resistance (drag) increases with the 2nd power of velocity. at slow speeds (low turbulence) differences in velocity are less significant, but in the 70/75 range, the difference in drag, and therefor MPG, can be large. i wouldn't totally discount the speedo "error" as a marketing device for owners to achieve seemingly better MPG, and even faster apparent acceleration times!
curious though - don't Germans pride themselves on always being on time? or is that just the Swiss-Germans? they still drive BMWs too though....
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