I've observed the same thing - the computer "average" clearly drops about 1 - 1.5 mpg from the first half to the second half of the tank. I've been getting 400+ miles from each tank, and even on a long trip in same conditions I'll observe this phenomena. It doesn't bother me - I record my mileage using hand calculation, not the computer - and my overall average mileage has been very consistent. It must have something to do with the starting point or assumptions the computer uses for it's averaging.