I use an Edge 530 and a Forerunner 920xt.
I have Continental GP4000 s2 700x25c
My Edge 530 is set to auto calibration (value 2134mm).
My 920xt is manually set to 2105mm
Interestly, I always get higher distance with my 920xt, even if the circumference is set to a lower value...
Anyone could explained that?