Try determining it by hand -- miles driven divided by gallons pumped. I suspect you'll find that your computer is 2 mpg optimistic.
Agreed. I have an '03 Sport V8 and just did a 300 mile round-trip on the Interstate with the cruise at 65. The computer said I was getting 20.5 mpg; the actual was 18.2. I filled up at the exact same pump on the way out and back at home, letting the pump go off automatically.
Is there any way to get these computers calibrated more accurately?