Welcome to BMW !

My 3rd one, and this one's the closest to the real speed, at +1.5 MPH (3KMH).
Each time I change to a new BMW, I use 2 different GPS to "calibrate" the error level.

I do 3 readings: (30MPH, 62MPH, 75MPH).
Then an average and I know at what speed set my cruise control.

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2010 X5 35d Black - Full (current)
2012 X1 28i (retired, won't be missed)
2007 335i sedan (retired, and missed)