IRR
But I have been told off by ATC before when their mode S readout showed I hadn't bugged 160k
I would be "surprised" if your Mode S reports "bugged" speed... If so, and this principle applied to an Airbus, ATC would have the delight of figuring out GS Mini
Our Mode S shows
actual IAS (and other parameters). The only "bugged" parameter it shows is Altitude/FL.
See
FlitePartners Ltd - Mode S Elementary & Enhanced Surveillance (EHS and ELS) for a list of "Enhanced Mode S" parameters...
NoD