I keep track of the miles covered for each tank of fuel but am not sure how this is entered onto Fuelly. Lets say I filled up today and I know I covered 400 miles on the tank that just emptied. On the Fuelly site, would I put in the fuel details from my previous fill up receipt? Otherwise it will be telling it that I covered 400 miles with the details of what I just filled up with, rather than the correct amount/cost of it previously?
So as I filled up today, I will be putting in my previous slip and keeping this one until I've emptied the tank?
I think you're kind of overthinking it. We run the stats on your driving based on your last distance and last tank full, by difference. If you fill up all the way each time (and in a similar manner, like don't top off) then we can assume your previous 400 miles used up today's fuel to refill to the top.
You can use Fuelly however you like--we think the important thing is to come up with a system that works for you to track your fuel economy. If you want stagger fuel-ups like that, that's fine. We didn't intend for people to do that.
We built Fuelly to work just like a notebook in your glove box. After you fill up at the gas station you jot down how far you went and how much you just put in. You do some quick math and you have the MPG for your last tank by calculating the difference from a full tank. This is a rough system, not rocket science. So the bigger picture is more important: how is my vehicle performing overall? The individual MPG values for fuel-ups are mileposts not the destination. So the fact that the exact fuel you entered in one spot doesn't match up with the exact fuel for a MPG calculation doesn't figure into how we set things up to track here.
I know there's the idea that once you're entering numbers into an application you can have precision tracking of some kind, but that just won't happen with this method of tracking your fuel economy. Fuelly is just a notebook with graphs.
I like to compare fuel brands so I use the method suggested by 'pb' here...my mpg figure isn't for the current fill up fuel, but for the previous tank full (i.e. the fill up I detailed on the row below) if that makes sense.
I usually detail any mitigating factors such as traffic jams, weather, lead foot driving etc so I know what caused my mpg figure to be so high / so low - when entering a fill-up I am detailing the driving and conditions from the just used tank to track my usage. Works for me!