Chitownblue

I recently purchased a 2005 4-cyl XLE, and have been extremely happy. I am concerned about the difference between the trip computer's calculated "average MPG" and my calculation (total miles driven divided by gallons at fill-up). After (3) fill-ups, the computer estimates 31-32 MPG and my calculation is around 26 MPG. I live in the Chicago suburbs, which requires the use of low pollutant gasoline. Does anyone have this same situation, and has discussed this with their dealer? Thank you for your help.