The dial on the PFD is most likely the Angle of Attack (AOA) indications.
Here is a short extract from AERO Boeing Oct 2000:
"The AOA indicator on the PFD shows an analogue and a digital display of the AOA. When landing flaps are selected a 3 degree wide green band is visible, it is centered at an AOA equivalent to Vref+5 (assuming nominal gross weight, mid CG, no sideslip, a stabilized 3 degree glide skope-thrust level and no system error). Normally the band accounts for expected variations."