I decided to quest for 700 miles for a tank (and by the way, it's gonna be close, I have about 1/4 tank left and am at 565 miles).

Now my lifetime average MPG is around 43mpg in my 05 HCH. I swear I have no idea why it's so "low", I do all the special tricks and a LOT of the time I'm able to cruise at 70+ mpg.

Anyway, I decided to try for 700 miles (I do have two 600 mile tanks). To do this, I completely filled my tank, I mean all the way up to the filler hole (ya, I know, cheating).

The first 100 miles I was at 54mpg average. At the 200 mile mark, total average was down to 49.9. By the 350 mile mark... going back-and-forth between 47.3 and 46.9. It has stayed there until 550 (where I am now).

When I saw it going down I decided to reset the other Trip meter... for the last 300 miles (the 200-500 mile mark on the main trip meter) I've gotten only 42mpg.

I promise my driving habits have not changed during this tank. I've driven on very similar roads, the ambient temperature is very similar (60-70 degrees), etc.

I've also found this to happen in the past.. the mileage starts at the beginning of the tank SO high.. then by the end of the tank I end up averaging 42-44mpg.

What is up with that?

7 - 600-mile tanks... farthest on "normal" gas fill= 643miles

And ONE 735-miler!! (cheated, over-filled tank )

Have you compared hand calculated numbers (miles/gallons) to computer's result? I can't think of a good reason for mileage to truly depend on tank content, so it would be a good idea to test the hypothesis that the numbers on the readout are incorrect. Next time you start with a full tank, drive 200 miles and record both the display readout and the hand calc. Fill the same way and repeat at 400 and 600 miles to check for any trend with respect to accuracy. After you do this (or maybe before, but obviously not at the same time) you might want to do the same test for filling up the tube vs. letting the pump cut off on its own. If there is a difference, maybe something screwy is going on when you overfill the tank.

Good luck!

Seems logical that it would go the OTHER way... A full tank of fuel should weigh about 80 pounds. As the fuel is consumed, the weight & load goes down. Not a lot, but it could make a little bit of difference.

I have no idea why your results are opposite to this!

I agree with ana, I would think so too....

FYI look down on bottom of page under similar threads... Prius folks have the same thing and there's a thread in a general section about it also.

7 - 600-mile tanks... farthest on "normal" gas fill= 643miles

And ONE 735-miler!! (cheated, over-filled tank )

I've noticed that I usually get a small "hit" in MPG when I start out. My temps are in the 50's now on the way to work. Even when I drive extremely slow and accelerate rediculously incrementally, I lose a few tenths of MPG. If this happens to you also and you make many small trips - it could have a cumulative effect towards the end of your tanks. Good luck and keep trying!
Regards,

Don
Best tank 67.5 mpg (555.5 mi.)
Best trip 91.5 mpg (42.2 mi.)

It could be related to the onboard computer software. You may be averaging the lower mpg number for the entire tank, but as with all statistical analysis, the larger the sample the more accurate the results. So, when you have gone through a full tank the computer has a larger sample to use to calculate mpg. One way to test that is to drive until you have used about 1/2 tank and then do a manual calculation and see if it is lower that the FE meter's reading.

Good luck,

Mark Owens

Hi if thats the case I would fill up when you get to half a tank.

Which half do you use first?

That's easy . . .

The half that gets to the engine first.

Need fewer troops to support.
Drive a hybrid.
Best tank 71.65 MPG.

Before this car I spent two years learning hypermiling on my 2004 HCH1.