To get the MPG to show after both a partial fill and the following fill-up, I deleted the partial completely but added its gallons to the gallons for the following fill-up. It should now have a "total miles driven" for a "total gallons used" at a fill-up, so I think the calculated mileage should be perfectly accurate. Is that correct?? Assuming it is accurate, does doing this cause any problem with how Fuelly does anything? THANKS.
Fuelly is using a full tank as a baseline for individual fuel-up MPG calculations. If you combine fuel-ups this way I think you'll find the MPG calculation will either be too high or low compared to your normal fuel-ups because your "tank size" will be too big.
It shouldn't impact your overall MPG because partials are counted there. But you'd be trading the no MPG calculation of a standard partial for a fairly inaccurate calculation. And you lose some accuracy in recording how you really fueled-up. So I'm not sure what the advantage of doing it this way would be. If it works for you though, that's ok.
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.
3snakes is right, there is no reason not to calculate the MPG on a fill-up after a partial fill up. Simply add up all the fuel added since the last complete fill-up and divide by all the miles since the same fill-up. You just have to realize that the MPG value for a fill-up is not the MPG since the last (partial) fill up, but since the last complete fill-up instead.
Like DTMAce I used to use a spreadsheet to track this stuff BF (before Fuelly) and that's how I calculated my MPG column. The thing is when you plot a graph of the MPG over time, you have to treat the partial fill-ups as missing data (not zero) and omit them from the graph.