Partial fill up problem
 

The partial fill-up system does not work well umless I totally misunderstand. E.g. I do partial fill up and check the box, no MPG appears which is expected. Next time I fill up completely and don't check the box, still no MPG. Why? Only on fill up after that is MPG given. And manually calculating the MPG given after partial fill up is to me incorrect. Can anyone confirm? Is the system supposed to add up the total fuel and total distance during partial fill ups or does it ignore non fill-ups?

Once marked, Fuelly won't calculate MPG for that fuel-up or the next. However, your partial fills will contribute to your overall average MPG.

Fuelly's calculations rely heavily on you filling your tank to the top because it uses the difference from full to calculate fuel economy on individual fuel-ups.
More info here: Why do I have to fill the tank all the way up every time I buy fuel?

Why not just add the previous partial mileage and gallons consumed to the next fillup to get the number for the two combined? I wager that's standard practice for most anyone who tracks their mpg on their own.

If I just need a few gallons to give me time to get to my preferred gas station, then I want the mpg for those few gallons and then the full fillup when I next get one.

+1

I don't understand the reasoning behind "suspending" the calculation of MPG. If I start with a full tank, add only 5 gallons for the next three years, and then fill the tank, it's perfectly reasonable to still be able to calculate the MPG over that time. We know the total miles traveled and the amount of fuel used between the full tanks.

I agree with how the system currently works. Assuming you are driving to completely dry empty, and then are adding 5 gallons each time, it's an incorrect value being returned. If you're lucky enough to be able to hit empty dry at the pump each and every time, though, you might want to consider investing in lottery tickets. Otherwise if you're not dry empty, you might have used .5 extra or .6 fewer gallons every few fillups and eventually you will just run dry while having also gotten incorrect readings for the last ... 3 years.

Calculations don't work based on the amount you put in, it's about hitting the same starting point as the previous time, or as practicably possible, in order for the calculation to be correct.

I believe you completely misunderstood my point.

Assumptions: 15 gallon tank, initially full.

Drive 250 miles
Add 5 gallons, partial fill - no calc
Drive 100 miles
Add 5 gallons, partial fill - no calc
Drive 100 miles
Add 12.5 gallons - CALC NOW

There is absolutely no reason to have to do a SECOND full tank in order to get MPG. 450 miles driven on 22.5 gallons of fuel. 20MPG.

You started with a full tank and mileage can be calculated IMMEDIATELY upon the next full tank, regardless of how many miles were driven in between and how many gallons of gas were added. All that matters is that you only calculate between full tanks, adding everything in that occurs in between.

Why do I have to sit down and do the math instead of Fuelly doing it for me in this scenario? Why does there need to be a THIRD full tank (started with a full tank, added a full tank, but it won't calc until the next full tank - and only if it occurs immediately after the second full tank)?

I understand not being able to calculate the MPG for a partial fill... Makes complete sense because you don't know how many gallons relate to how many miles. But, once you have a fill up (full tank), you can calculate MPG using everything that occurred since the last fill up. And, Fuelly won't do this.

Yup, that makes more sense. :)

I agree. You have all the information you need to calculate the MPG when you finally don't check "Partial". What I've done is to not enter the partial and add the gallons when I do fill the tank. I shouldn't have to do this!

wow, I couldnt figure out what was going on with my fuellings of which I have several partial ones recently, until I came to the forum to see what I was doing wrong.

This is a MAJOR shortcoming and really renders this application useless!!

this really needs to be fixed.

Not being able to calculate mileage with a partial fill makes sense - there's no way to tell how many of the miles driven by the last fill up used how many gallons of the partial fill. If you drove 200 miles and put in 1 gallon of gas, what does that mean for MPG?

The issue is that the partial fill(s) are not added together from the last fill up to the current fill up with all miles in between used to compute an average MPG over that duration.

I feel there should be a "Current MPG" that shows the most recently calculated MPG based on a complete fill up. If that calculation also takes into account partial fill ups, it should appear in a different color or have some other sort of "asterisk" to it.