I can tell you that i had my 12 vegas speedo updated and those displays were working immediately with out any real intervention from me. All i did was scroll thru all the readouts make sure they were working.
have you tried this:
Average Fuel Economy (Accessory)
This mode displays average fuel economy since being reset. To reset, toggle to this mode.
Press and hold the mode button until the meter resets.
'12 Vegas, 78 Bonneville, 69 BSA project.