I read somewhere that the speedo cannot be accurate throughout the speed range. Meaning that if it is right at 100 mph it will most likely be wrong at 30 and vice-versa.

Also, you must consider the condition of the tires and how much tread has been worn off.

I used to check my speedo in all my cars against the mile markers on the highway and kept a conversaion factor in mind for each car.

Now, I just use the gps monitor to tell me the speed I am traveling - although I know is also off, but at least it is not dependent on the car/tires.

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2006 X5 Sport 4.4i