Front what i have concluded it can use either abs sensor for speedo. The Non-Abs models have and abs sensor on the rear wheel only. I have broken my rear wheel abs sensor, and my speedo still works.
So i guess it can use both or either.
But it has no way of knowing what the diameter of the tires are, so I'm guessing it would be inaccurate.
2010 F800gs (Mine) 2011 F650gs (Wifes)