BobM2
There is no "speed" input to the transponder. ATC gets groundspeed on the target from their own computer.
it the mode S transponder has enhanced surveillance it sends out more data than you might be comfortable with including:
Airspeed
Ground speed
Vertical speed
Track
Bank angle
Desired altitude ( yes it rats out what you've dialed in or have in your flight plan)
Heading
Bout the only thing it doesn't spit out is if the pilot had the turkey on white or the ham on rye for lunch.