I've heard of this story! The same guy, who flew the SR-71, also tells of another goodie.

"SoCal centre, Cessna 172 NXXXXX request a groundspeed readout?"
"78 knots"

"Beech Baron NXXX requests our groundspeed readout as well?"
"125 knots"

On comes a smug sounding F-18 pilot "Dusty 52, ahhh, request our groundspeed readout?"
"..525 knots"

Couldnt resist they said: "Center, Aspen 10, request our groundspeed readout"
".....1712 knots."

No one else asked.