I don't get what you mean, maybe it's because English is not my main language, maybe it's because I don't get it all together but, why would it be more accurate to fill up from empty to full than from full to let's say half full and then back up again to full?'

If I have 40 liters in my tank, I then drive 200 km. and fill it back up to full and see how much I have used - (let's say I used 10 liters exactly) - then I know that my REAL WORLD fuel consumption is 20 km/l ... why would this NOT BE 100% accurate?

(The only reason I can see - and that would be the same no matter if it was empty or not, is to use the same petrol station and pump (there can be a little bit of difference between each pump)

I fill my tank virtually every time I add fuel and enter the information into fuelly, a fleet management software and excel for each fuelup but how I use fuelly is irrelevant. If you read my previous post properly then you will see that I already stated that full tank to full tank is the only way to be accurate.

As for fuelly being 100% accurate - there is always some miniscule inaccuracy due to the rounding and /or significant decimal places (aka truncating error) used in calculations. With every computer and calculator when doing floating point calculations if you divide number (A) by number (B) you will get a result (C). Now if you multiply (C) by (B) you should get the result (A) but with the aforementioned errors you can get a slightly different result. Anyone who has experience programming where floating point calculations are used will no doubt have experienced this at some point in time. That being said, Fuelly is very accurate but so is the fleet management software I use and so is excel but there have been occasions when the same information entered into all three return different results because I choose to only display 2 decimal places and each one uses slightly different rounding or truncating algorithms.

As such I stand by my statement that fuelly is very good but the best.... Hard to say.