Has anyone noticed that their MPG numbers go dramatically downward as you reach the end of the tank?

I don't have enough evidence to support my theory yet, but it appears that everything is fine and dandy with MPG until the gas gauge drops below the 1/2 mark. Then, no matter what I do to increase the MPG figure from that point to the 1/4 mark and lower, I keep *losing* MPG instead.

If this really is true, then that means inorder to maintain MPG numbers, I will need to fill up at 1/2 tank, which doesn't make a lot of sense to me. Anyone else's input?

I notice similar effects in my 05HCH. My theory is that as the tank passes the 1/2 way point, the computer accumulates more data points. More data points = more accurate results. If you don't reset the trip/MPG odo each time you fill up, you will notice a more stable MPG reading with time. When I first fill up my tank and reset the trip odo, I notice mpg reading anywhere from 50-70mpg.

If you have an A/B trip odo/MPG meter, try keeping one of them on long term without resetting it each time and the MPG data will eventually smooth out.

Ralph is right. The computer is making approximations.

The only way to accurately calculate a change in MPG per tank is to use the number at the gas pump. Even if your odometer reads higher or lower, your % change in MPG will be accurate.

Just recently my HiHy read 40.4 mpg over the first 10 miles after fill-up. I can assure you that at my next fill up, my calculated mpg will be around my usual 21mpg.

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.