Nice one.
Your long-term fuel fill logging method is far better than a single journey full tank to full tank method. But still, influenced by the time of the day you do the fill.
Below is a study of fuel temperature profile (in the tank) taken in 4 consecutive days (0:0 means midnight, 1 notch is 3hours). You can see in the figure that the best time to do the fill is around 6:00 AM.
Full-tank-to-full-tank method is very inaccurate especially if there is a large volume of fuel remaining in the tank. This is because petroleum based liquid expands by 1% for a 10 to 15 degC temperature rise.
In a single journey, and let say fuel temperature inside the tank increases by 30degC due to the hot fuel from the return line; the volume of fuel could increase by 2 to 3% of volume.
So for example if someone driving diesel vehicle with tank capacity of 75 liters and traveled 400km, then at the end of his journey he just filled-in 24 liters of diesel at the station (means 75-24 = 51 liters remaining prior to the fill). He can easily compute 400km/24liters = 16.7km/liter then say wow great!
Taking into consideration the thermal expansion of let say 3%, we can remove the bubble (bloating) from the 51 liters remaining in the tank by
51/1.03 = 49.5 liters should only be the remaining volume inside the tank. This means that 25.5 liters should have been the volume filled to make up the 75 liters full tank capacity.
This also means that 400km/25.5liters = 15.7km/liter is the corrected average fuel consumption.
Kaya mapapa tsk tsk na lang dito sa method ng ating DOE pa man din. Only 333 kms traveled and many are having over 50 liters of fuel remaining inside the tank occupying more space (when fuel becomes hot) that should have been filled by fresh fuel from the station.
DOE Completes Record-Breaking Fuel Economy Run
http://www.doe.gov.ph/news/electrici...may%202008.pdf
Sana itago na lang ng DOE sa pinaka baul / archive nila. Kawawa naman yung gagamit ng DOE results as a reference baka mapagtawanan lang lalo na sa foreign forums.