A DME equipment with a groundspeed readout
will calculate groundspeed by using rate of change of range.
Obviously for the groundspeed readout
to be accurate the slant range must be as close as possible to the plan range.
The only time that will occur is at
long ranges from the DME.
Similarly, the rate of change of
range will only produce an accurate groundspeed when flying directly towards or
away from the beacon.