Actually if I may...
The only thing really affected by doing this method, is that your total gallon size, fill up cost and gallons per fill up is affected, as you are ending up with (usually) more in the actual units of each value than you would if you always use a per tank fill up.
However. MPG should still be spot on, as this particular method would at least allow you to have a MPG for the overall of the partials and the final fill up (that HAS to be a full tank) added together. It would be an average of those partials + the full, but it would be a more accurate way of individually measuring partials as a group. Obviously you want to limit this from occurring as much as possible.
Keep in mind, this could throw the averages for your cost per fill up, etc as it would be higher than they should be. I think that is what PB is getting at. Fuelly still calculates your overall with the values, even from a partial, just that the math is very complex for doing partial MPG individually, as Fuelly has no way to calculate the correct MPG. It is supposed that if Fuelly always knew exactly how many gallons your tank held, it could approximate, but it still would not be as accurate as full tank to full tank measuring.
My excel sheet that I created to import data from Fuelly is setup to do what you are describing. It can add up to 10 partials in a row to the next full fill up. But since I have never entered a partial fill up on my vehicles, I do not really need it. I know it works, as I have tested it, and its pretty spot on.
Good luck!
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