Ive had tanks as much as 5.5 and 5.8 MPG off, but my average over 16 months show only 1 MPG difference between FCD and my manual calculations. Being off more than 5 MPG happens on rare occasion, just the combination of things, hard to exactly pinpoint the problem, since no one really knows how the computer does it's calculations drive to drive.

Don't expect it to be that far off all the time. If it does THAT, then your car might have a problem.