I also find the non-computation of MPG on a partial fuel up annoying. Why can't Fuelly add up the gallons since the last "Fill", and use that odometer reading as well? Sure, you still won't get MPG for consecutive partial fills, but you will get an average MPG between all fills. Not just consecutive fills, the way it is now.
I often do these intermediate partial fill MPG calculations myself, and put the result in my notes. When I review my log, putting the cursor on the note field tells me the intermediate MPG, and other details I like to track.
I have a couple of unusual reason for partial fuel-ups. I have an F250 Super Duty Diesel pick up with a 58 gal tank, and it is capable of a minimum of 600 miles range even when towing. I pay cash for fuel, so a $100 bill will give me 325 miles towing or 500 not towing. It's convenient to buy an amount that I know will fit in the tank, and not have to go back in for change.
The other reason: I live in Los Angeles, and frequently my destination for a trip is out of state. I take on just enough fuel to make it to one of my 'regular' out of state fuel depots in Arizona or Nevada. I despise the State of California with all its taxes and rules, so I purchase as little fuel as possible in Cali. In 23,000 miles, less than 12% of the fuel consumed has been purchased in California. I'm doing my part to Starve The Beast.
58 gallons, 600 miles?! It really is a different planet over there!
You have to understand that things are different over here. We own large boats, campers, and other toys that don't seem to be popular in the UK that require large thirsty vehicles to pull them. Driving a little underpowered econobox is just not practical for a chunk of the population in North America. I owned a small car at one point (diesel VW Golf) that got great fuel mileage but just didn't suit my needs other than getting some groceries or driving back and forth to work in decent weather. I for one would rather have a pickup truck with the capacity to haul and tow things instead of having to to rely on and pay someone to deliver anything and everything that wouldn't fit into a car.
It returns the 600 mile range when towing 6,000 pounds, and the truck itself weighs 8,000. So 14,000 total weight, 600 miles on ~50ish gallons.
It goes almost 1,000 miles on a tank when it isn't towing. On a recent cross country interstate trip it returned one leg of 20.14 MPG cruising along at 77 MPH. This thing is huge. It's like sitting on your living room sofa looking out at the view. My 8,000 Lb diesel truck gets better mileage than any of my 5 petrol V6 S-series blazers and pick ups which are usually in the 16.5-ish range on the ethanol crap for gas we are forced to buy in California.
Thanks for the interest and comments.
Back on topic: I posted here as another point of view as to why I'd like to see Fuelly use a modified algorithm to calculate MPG between full fuel-ups rather than only between consecutive full fuel-ups the way it is now.
I guess aerodynamics play a big part in a trucks economy too. For example a V8 diesel in a car like an Audi, Mercedes or BMW will easily return 40+ MPG. And I guess the gearing on your truck is probably designed with towing in mind too.