I have a huge error in the MPG indicated in the insturment cluster vs calculated with a dead nuts on odometer.

I just got back from a 3,000 mile run and had a chance to check out about every engine perameter. I have an 18% mileage error. The car calculates out like 32 MPG and my calculations are 27.

How is the car calculating the mileage and is there any way to make and adjustment to correct this errror?

PS: The LS6 knocked down 32 to 34 MPG at a steady state 80 MPH cruise. Not too bad! ;D