ATTROCIOUS FUEL CONSUMPTION ISSUES

OK Guys

What is going on

I am averaging 21.5 Litres / 100 K's

My Worked LS1 Tonner only uses 11/100

I plugged in my OBD11 reader and found 3 codes.

I was able to clear them however they came straight back.

If anyone can dumb this down for me i would appreciate it.....................................