At LukeSkyTodler:
What do you mean, the speed the ATC can "see" is the selected speed.
Would they not read out the GS of the aircraft?
Hence they ask what the indicated speed is, or mach number?
Would a mode s transponder show the selected speed?
Many reasons to use V/S though, one of them to slow down your V/S when approaching your target altitude to avoid TA's or RA's to other traffic around.
Rgds