Don't know about the hidden menu, but I believe you will find your speedometer optomistic by about 8%. This used to be the case and has to do with European regulations which state that the spedometer's error can never be such that you are actually travelling faster (as opposed to slower) than the indicated speed (speedometer).

You can do a quick check of this by setting the cruise control on the highway when the speedometer is at 60 mph then reset the average speed on the on board computer. The on board will show a faster speed than the speedo. At least this has been the case on every BMW to the early 2000's.