@Draigflag, if you wanted to calculate mpg without using full tanks, how would you do it better (given that you can't get an accurate mpg calculation per tank)? Noting that Fuelly already shows a 'total mpg' calculation based on total cumulative fuel used & miles driven.
Im not suggesting there's another way, im merely stating that almost everyone I know only put a couple of gallons in at a time due to the expense. For a lot of people here filling up could cost them half a weeks wages if not more. But I do live in a rural country so wages arent especialy high.
That "couple of gallons at a time" implies a lot more trust that I have in the accuracy of the fuel gauge. I much prefer to fill the tank and set the trip odometer to zero as a fuel gauge backup. Even though the initial fill- p is expensive, your driving habits are probably not going to change a lot, so you won't burn any more fuel in the long run. The question does occur to me, though - if filling the tank is too great an expense, how does one afford the vehicle in the first place?
OK, I have a suggestion yet I don't really recommend it. I was measuring the distance between fillups via the distance between the low fuel lights.
Teresa has a one-bit low fuel light instead of a complete gauge, and it seemed to be a good idea to use that as I thought it would be more accurate than trying to fill up to exactly the same level (and fail).
I was wrong, but not utterly wrong. The fuel level sensor is inaccurate too (influenced by the incline and acceleration), and now I think it's less accurate than my fuel ups - so I switched to the common fill-to-fill method.
The reason I mentioned it that the light-to-light method let me fill any amount I wanted without sacrificing too much accuracy. So it can be a solution too, just keep in mind that as soon as the light comes out (the gauge reaches a specific level) on anywhere but level road, it'll influence the result.
I did consider this too, mine comes on when there is 5 litres, just over a gallon left. I imagine though, as you say, unless travelling at a steady speed on a flat road, the time it comes on varies. Ive also noticed pressure in the tank can affect this. I do very low miles during the week and often have the warning light on all week! Ive noticed it can be on one day, then off the following morning which I persume is somthing to do with the pressure?
I can't rule out the pressure thing, the YARDIS' fuel gauge can adjust itself after going a couple of kms too. It could also be caused by slow reaction to up/downhills, though (I often park downhill at home, it can give a roll and a clean bump start ).
In Teresa's case it seems to be affected by the lean angle too (at least leaving her on the side stand for a while can light up the indicator early).