I check my car's speedo against a Garmin GPS. My current one is 2kph fast while my wife's previous one was absolutely accurate - very rare (not a BMW).
GPS speeds are from integrating successive positions and are accurate. Borrow a Garmin or Navman and compare them with your speedo. You need...
I know this is an old post but the ACC hasn't been updated obviously so my comment is still relevant.
I've driven about 50,000km in my 2014 M-B SUV with adaptive cruise control - Distronic - so I can compare the ACC on our new i3. The M-B has NEVER dropped out. It follows accurately through...