When filling up today to check my mileage I had an issue.
OK, I went to fill up and check out what my mileage was after using the P&G and EOC techniques. When I put the pump into my gas tank it stopped at .5 gallons and wouldn't discharge anymore gas. I thought that it was the pump so I held it back some until it would allow it to pump the gas in. I was able to get 2.66 gallons total in and it was topped off to the fill tube. I don't do this that often I was just caught off guard by the whole situation.
My question I guess would be is it logical that I only burned .5 gallons? I mean, I was able to squeeze 2.66 gallons into the tank albeit overfilled. I am trying to maintain my fuel log and this throws a huge wrench into things. I mean is it possible that I almost got 80mpg in 100% city driving?
You can't get accurate readings from 40 miles of driving; you need more miles to get accurate, reproducible results. If you check your average every 40 or 50 miles you'll see wild huge fluctuations and you'll never know what worked. Besides needing more data due to driving fluctuations (heavier traffic today, warmer air temp tomorrow, etc), pump inconsistencies make HUGE errors at that scale.
I understand being eager to know what's working. In fact, I fill every 200 miles, even though I can get 400 miles per tank. I get stable results if I fill at 200 miles. That's enough to account for inconsistencies in my driving, the road, the traffic, the weather, and the pump. Also, I pump at the same pump every time.
If you were parked uphill last time you filled, and downhill on this fillup, that can relocate fuel in the tank.
I like to run three full tankfuls and average all three tanks (a 3-tank running average) to see how my mileage is. (This is complicated for recording your mileage in a normal notebook, but simple to calculate with Excel etc.)