Partial Fuelup Calculations Improvements
Presently Fuelly does not calculate partial fuelup(s) and the next full fuelup. This discards fuelup MPG which can be calculate by aggregating partial fuelup(s) with the next full fuelup.
The Fuelly FAQ states: "How do I account for partial fuelups? If you can't completely fill up to the top when you're at the pump and you want to track the fuelup in Fuelly, check the box next to This is a partial fuelup. This will let Fuelly know that you have a partial tank. Once marked, Fuelly won't calculate MPG for that fuelup or the next. However, your partial fills will contribute to your overall average MPG." It is possible to calculate MPG which includes the partial fuelup. Simply add the partial fuelup(s) to the next full fuelup. This works out b/c the the number of gallons used in the partial fuelup(s) plus the next full fuelup is the number of gallons used to travel the miles for the partial fuelup(s) plus the next full fuelup. It may be useful to denote these (for example, with an asterisk) to indicate that partial fuelups were included. This provides better and more instantaneous MPG rather than discarding the partial fuelup(s) and next full fuelup. 
Yes, I had to do this on my fuelly tank 34 (it's two tanks added together). There was an attendant who insisted on fuelling up for me for this particular tank (never happened before), so it wasn't vented&brimmed as usual (probably 4 or 5 litres short).
Initially I tried to do it as a partial tank but ran into the problem that you found. 
Adding to what I just put:
I put the detailed figures for both tanks into the notes section, so that I would be able to unpick them in the future. Ultimately this is something which fuelly could do  if there is a partial followed by a full fuelup, the partial tank and the following one could be combined on the graph & an mpg figure could be produced for the second tank (based on both tanks together). 
We can't really do that because Fuelly's individual MPG calculations are based on the difference from full. Partials are not a full tank, and if you combine them with another tank, the size of full changes. So the individual mileage calculation would be off.
We know it's frustrating to not get an individual fuel economy number when you add a partial, but we don't want to provide misleading numbers. And combing full and partials would be misleading because the size of the calculated full tank would be too big. There's more info in this FAQ: Why do I have to fill the tank all the way up every time I buy fuel? 
Oh, and just to be clear, you mention in your post that "This discards fuelup MPG..." and I just want to note that partials do contribute to your overall average fuel economy. For your overall average Fuelly calculates your total fuel volume filled and total distance driven. So even though you don't get individual MPGs for the fuelups around a partial, they do contribute to your overall average.

"We can't really do that because Fuelly's individual MPG calculations are based on the difference from full. Partials are not a full tank, and if you combine them with another tank, the size of full changes. So the individual mileage calculation would be off."
No, that is incorrect. Fuelly already handles different "size of full" in the smaller "size of full", because no special handling is required. If someone drives to empty and fills up, this is recorded/calculated find. The next time, they fillup before going to empty, say with half the tank left. The "size of full" is smaller, but the MPG calculation is still correct. What matters is in the MPG calculation is that the amount of gas correlates to the miles traveled. This is still correct when adding partial fuelups to the next full fuelup. This changes the "size of full" larger, but is still mathematically correct, just as a smaller "size of full" is mathematically correct. "Oh, and just to be clear, you mention in your post that "This discards fuelup MPG..." and I just want to note that partials do contribute to your overall average fuel economy. For your overall average Fuelly calculates your total fuel volume filled and total distance driven. So even though you don't get individual MPGs for the fuelups around a partial, they do contribute to your overall average." Yes, I did fully read the FAQ and understand that it contributes the overall average MPG. "Last tank MPG" is a misnomer for what I described, so I used the the term "fuelup MPG" which is a superset of "last tank MPG". 
Correction to my last post: "recorded/calculated find" => "recorded/calculated fine"

>Partials are not a full tank, and if you combine them with another tank, the size of full changes. So the individual mileage calculation would be off.
I'm having trouble understanding the logic here. Obviously a partial tank is no good for calculating MPG for that tank. However one (or more) partials followed by a full tank should give you all the data you need to give a combined MPG figure for that subset of tanks. We've all read & understood the FAQ, but it's not relevant to this particular scenario. 
Example:
Tank 1 : 30 Litres 300 Miles Full tank Tank 2 : 20 Litres 300 Miles Partial tank Tank 3 : 30 Litres 200 Miles Full tank Tank 4 : 30 Litres 300 Miles Full tank If we look at the tanks individually ... Tank 1 and Tank 4 can be calculated OK (300 Miles on 30 Litres). Tank 2 is bad because we don't know the true amount of fuel. Tank 3 is bad because (without looking at tank 3) we don't know the true amount of fuel. However... What is being suggested here is to effectively combine Tank 2 and 3. Tank 2/3 : 50 Litres 500 Miles 500 miles on 50 litres. We get an accurate MPG figure for the combined tanks (although not individually). There is no missing information, both fuel and distance is known accurately. Fuelly is fine with this at the moment if we manually combine the tanks. Ryogajyc is suggesting that fuelly be able to recognise this scenario and supply a combined MPG figure for the series of partials & the final full. Now, it is true that this sort of sequencebased logic is difficult in SQL because SQL is focused on set manipulation and not sequence manipulation (I don't know if there is an equivalent to Oracle's ROW_NUMBER or Ingres TIDs in MySQL, but that would make it easier). 
Fuelly is based on the simple idea of one trip to the pump = one fuelup. Once we start combining fuelups and assigning an average across multiple fuelups we start to lose that simplicity and it becomes harder to explain. I understand what you're saying here, and I understand the frustration with not seeing a fuel economy number every time. But we'd rather err on the side of keeping things simple.

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