How does the average mpg work?

I don't find per tank MPG informative, especially as the tank capacity is variable.
Instead, I reset at the start my commute (or other trip).
Then, I'm comparing apples to apples.
In this way I improve my driving techniqe by noting the effects of the lower number of variables affecting my MPG.

 

what I find interesting in John1701a's mpg graph is that it totally flattens out after about a year. Just like the lifetime mpg on the mfd. I just reset the trip odo and look at the gas guage and I can see if something is drastically wrong. As it's all on a spread sheet if there is a drastic drop in mpg I'll just print it off and take it to the dealer. When people ask what kind of mileage do you get with that thing I look at the readout and tell them. I't as close as damn is to swearing. To each their own though.

 

I can confirm that an 06 does not reset after each fill up.

I like the GreenHybrid.com thing in my sig. I just get a recipt of every fill up and write my milage on it. I just put it aside and enter it when I get home. that is a good way to track your lifetime. It is about .5 MPG off of what my MFD says.