To know your ground speed while taxiing, set the right MFD to CDU. On the upfront controller, hit "function" then "Hack" this will set the CDU to POS (Position) mode. In the upper right hand corner of the MFD, one line down, you sill see the button to switch between IAS, TAS, and GS (Indicated Air Speed, True Airspeed, and Ground Speed). Just hit the button next to it to cycle through to GS. Now, Ground Speed will be displayed on the CDU right next to the type of speed selected. Best practice is to not exceed 21 knots on the ground in a loaded A10C. The other air speed types come into play when doing advanced IFR flight later on, but it's good to know how to do this early on.
