Too many variables. To be exact, which the end result should be, you need to do this with a same vehicle and fill the same amount each time. Other people have different driving habits, and their vehicles are different; some well tuned, some not. There is all weight differences to take into account; how often during the experiment they used cruise control (it does save fuel) during highway driving or not. Whether they have the correct tire pressures and speed variables, just to name a few. If you do your experiment the way you envisage, I don't think it will tell us anything useful. Sorry if I sould like a naysayer.
