Hi Arthur6!
I don't own a C6 but do have a C5 with the HUD. The way mine works is if you press and hold the "PAGE" button, you can scroll thru the pages in order: Speed Only, Speed and Gage, Speed, Tach & Gage, Speed & Tach, Tach only.
Gages can be scrolled thru by tapping the "PAGE" button (only when gage is displayed). They go in the following order: Oil Pressure, Coolant Temp., Fuel Level. This is right out of my '03's manual.
Hope this helps you out some.
Chris