I'm a little unclear as to what is done with the partial fillup data. Is it completely ignored? Or are the total miles and total gallons between fillups calculated to determine mpg?
For example: say you fill up, drive 400 miles, put in 1.4 gal. because it's close to empty and you only have $5 on you. Drive another 20 miles, get some cash and then fill it with 9.1 gal.
Wouldn't the calculation for the fillup be:
(400 + 20)/(1.4 + 9.1) = 420/10.5 = 40 mpg?
Or would it not calculate mpg for the second fillup and wait till the next fillup? That would seem strange to me.
Fuelly is not setup to calculate partial fuel ups in that method.
They do count in the overall however, so your averages, and overall mileage will reflect the total distance/units of fuel.
I myself have queried this very issue, as I had found ways to self calculate the differences so that they didn't have to affect my fill-up to fill-up mileage. In those instances basically your above example applies.
However to use it here, I combine the two fuel-ups, using the odometer (or trip) for the actual full tank fuel up, add the cost of the partial(s) and the full tank together and divide that by the number of total fuel units to get your price per unit, etc.
Bit of a pain, but its a way to deal with them if you don't want to report a partial here. The only problem with this is, the per tank costs won't be reflected as accurately, but the overall total and averages will.
The spreadsheet I use here, actually can work with up to 10 partial tanks in a row. They simply get added to the next full tank.
For the record however, I try always use full fill ups. Use the method of always half full or full. That way it doesn't cost as much to keep it full, plus you always have plenty in the case of an emergency.
Fuelly calculates an individual MPG for a fuel-up based on the difference from full. So without a full tank to work with, there's no way to make that calculation. So while you won't get an individual MPG calculation for a partial, it does contribute to your overall average MPG. For that calculation, Fuelly uses total distance and total fuel filled.
Why not use the partials in some way to calculate an individual MPG? Because there's no way to know the tank size without a full fuel-up. Imagine adding three or four partials in a row—your "tank" size would grow with each fuel-up. And the resulting MPG calculation would be misleading. We feel like it's better to err on the side of accuracy and only provide individual MPG calculation when those are based on a preceding full tank.
OK, to sum up: fuelly doesn't ignore the partial reports, it just doesn't calculate the individual mpg for the partial fill, or for the complete fill after a partial. It waits for two complete fills in a row before calculating an individual mpg for the second fill. The overall average mpg uses total distance and fuel reports, including partials.
And yes, the reason I've never experienced what happens with a partial fill is I've never gone to a station without completely filling the tank - that's what credit cards are for, after all.
I once considered a partial fill due to the tank being near empty and the nearest station charging 12 cents per gallon more than the station I usually use. In the end, I decided against it and did a complete fill-up.