Tank Capacity
DanMan,
No, what I was saying is to try to put the amount of gas in that the computer says you used, based on the same computer telling you what the average for the tankful just burned. As an example, on my last fill, the computer showed that I had averaged 60.1mpg for 545 miles. If you divide 545 by 60.1, it shows that 9.068 gals were used. (rounded off to match pump configuration) I was able to put 9.293 gals in the tank. Now, I probably got it slightly fuller than on the previous fill, or maybe the temp was slightly different, as the time was approximately 1 hour earlier in the afternoon. This gave me a calculated average of 58.6 mpg as opposed to the 60.1 shown on the mfd. (BTW, the last pip had been flashing for over 20 miles, apparently with at least 2 gals still in the tank.)
The best way to figure gas mileage I think is over an extended period of time, so that you allow for variations in temperatures, levelness of the driveway at the pump, how full you actually are getting the tank from time to time, etc.
You also have to be very careful when topping off, which is why all the pumps where I live warn against the practice. The Prius will sometimes make a gurgling sound, which I take to mean it is very near full. If gas sits in the top of the filler neck for more than a few seconds without draining down into the neck, I consider it full.
I apologize for the length of this posting, and hope it clears up what I was trying to say earlier.