Range and L/100KM accuracy

I suspect this topic has been beated to death but in case it hasn't Ill try. I'll explain to you in step format what my problem is so its easy to follow

Step 1) Fill up gas tank full, and I mean COMPLETELY FULL right to the very top
Step 2) Start car. Range: 570KM (Should be more like 800 or 900)
Step 3) Head onto highway
Step 4) Set cruise control speed: 110 KM/h
Step 5) Reset Trip A and B odometers to 0
Step 6) Maintain speed for 160 KM without touching anything
Step 7) Observe Average...

Range and L/100KM accuracy