Ok, I was reviewing this a bit more. I can see where there could be issues calculating this into your average price per mile and price per fill up.
I am assuming you are using total mileage divided into total cost to get this figure. If that's the case, this should just still work as is.
Now with regards to the average price per fuelup. Since the average is supposed to reflect true fillups, it would remain correct. The partials could be ignored.
Or, you can calculate those in as separate fillups to get your average, but they would lower it. Price per Fillup is simply about how much money you spend on each stop for gas. It doesn't have to matter how much gas is put in for it to be correct in the overall scheme, but this may be a preference, one way or the other. While it is true I don't know the exact math you guys have setup here, it can be figured out, and still keep the same theme and goal of the site. Ok, I'm done I think. LOL
It is true that partial fill-ups sort of wreak havoc on the whole idea of tracking average distance between fuel-ups, and average cost of fuel-ups. What would be interesting (though labor-intensive) would be to compile a list of tank sizes for various vehicles (which actually, individuals can easily do, to get an idea of what's happening). If I know, for instance, that I am averaging 32 MPG all-time, and my tank is 12.2 gallons, I know that my average distance between fuel-ups should be 32 * 12.2, or 390.4 miles. Ditto for average cost per fuel-up, based on the average price per gallon. I have tracked my own mileage so carefully, that when the 2-gallon gas light comes on (and I know I have about 60 miles effective range), I have felt comfortable traveling up to 40-45 miles. Talk about freaking out your passengers!