Not sure yet sure, but like longjob said it seem to depend of the speed. I earase my data but I think it was something like 15m by every 3km. Maybe more... Today I tried GB-580 and 625xt on my scooter at high speed for 5 km and there was only 2 or 3 meters difference.
I will soon try 625M, GB-580 and 625xt at slow speed. All on my lateral bike bar. If there is a difference, I think it might be firmware related.