pb
Jan 7, 2010
3:26 PM

Because Fuelly makes its MPG calculations based on the difference from full. So if you don't fill the tank all the way up, there's no way for Fuelly to calculate the MPG for that particular tank or the next one. Fuelly needs a full tank to work from to get that fuel-up MPG. That's the basics, but there's a longer explanation in the FAQ that demonstrates why: Why do I have to fill the tank all the way up every time I buy fuel?

Keep in mind though that your partial fills do contribute to your overall average MPG. That calculation is based on total distance driven and total fuel volume, so it does still help to add partial fills into the mix.

Yes, I understand that. However, you could sum the total miles and gallons between the fill-ups, including the partials, and I think it would still work.

For example:
From a full tank, I drive 180 miles, and buy 3 gallons of gas. This partial tank would be impossible to calculate, of course.
From that partial tank, I drive another 30 miles, and then fillup with, say, 8 gallons. Taken by itself, this tank is also impossible to calculate.
However, if you add them together, you would have driven 210 miles on 12 gallons of gas. This calculation should be perfectly accurate.

This is the exact same calculation that is done for the average MPG, except it's taken over consecutive partials and the next fillup. This means that several consecutive partials could also contribute to the MPG for the next fillup, instead of just the most recent. In fact, they would have to for this idea to work.

Am I missing some knowledge that would prevent this from working?

This is precisely how I deal with the "partial fill-up" problem in my own spreadsheet, and even go one step further, to avoid having blank entries:

You know that you went 210 miles on 12 gallons; that's 17.5 MPG. So, I go back and fill in the MPG for the 3-gallon "tank" with 17.5, and the 8-gallon "tank" with 17.5.

Now, I also like to track how different brands/octanes do over time. Obviously a partial fill-up messes things up here. But now I can go back and run the calculation, including 3 gallons of brand A at 17.5 mpg, and 8 gallons of brand B at 17.5 mpg. Obviously, it's not perfectly accurate, but it's close enough to get an idea of what's going on, and considering that I've only had three partial fill-ups in the last three years (precisely to avoid throwing my calculations off!), things smooth out nicely over time.

Incidentally, I think we're missing the greater point here, to wit: 3+8 = 11, not 12. =P

Idea's still the same, but the numbers are uglier.

Huh!
This is why I'm not going for a math minor :) Being a programmer is enough for me.

pb
Jan 11, 2010
9:56 AM

I see what you're saying, hufman, you'd like an average that includes partials for your next full tank after a couple partial fills. That's one way you could do things, but it's not the way we chose to set things up. We're sticking pretty strictly to the idea that fuel-ups are discrete individual activities, and when we give an MPG rating for a fuel-up, we want it tied as closely as possible to that idea.

But like I said, you could build a fuel-tracker that works the way you've outlined here and that'd be fine. But we're happy with the way we have things set up.

Ok

DTMAce
Jan 12, 2010
7:24 AM

There are one of two ways to deal with this as I see it.

First off, I find that using the odometer, not the trip works better for me, and I will show you why.

Option 1:

If you have to do a partial fillup and you are using the odometer, keep track of the gallons put in during the partial fuel up. As for price, that's where it gets a little tricky. Keep track of the price, but you will have to average (add all the different prices together then divide by the number of stops involved) When you finally do a true fillup, make note of that odometer reading. You can do this with a trip counter as well, just don't reset it till you do a true fillup.

Then log into fuelly, put in your odometer reading from the true fillup, then your average price per gallon and the total amount of gas you bought from all the stops. It will make it look like you have a huge tank, but it will also give you the correct MPG. The pricing will not be as accurate due to the total/averaging, but its going to be very close.

Option2: The staff here on fuelly could make a change. I just tried playing around with the partial vs full to see what the site does as I have yet to do a partial fillup. I see now that it does nothing at all. You get 0 MPG for the partial, then 0 again for the next fill up. I think with some simple math, this could be fixed akin to above. If you set up the partials to track the odometer/trip, the fuel volume and the prices, there should be a way to implement a similar math technique as I used above. Try setting up something like this:

Odometer use:

Person goes and buys a partial tank of gas, lets say 5 gallons. Their last reading was 10,000 miles and at this partial it is 10,300. Lets also assume they paid 2.499 for their gas. Fuelly then stores this info into the data base.

Lets say their next reading is 10,400 miles. This time they fill the tank full and it takes 15 gallons. The price ends up being the same at 2.499. Log into fuelly put in the above information. Fuelly's math then does the following:

Adds the mileage between stops together. (300+100)
Adds the gallons from both stops together. (5+15)
Calculates the MPG from this information and only displays it on the full tank fillup entry. The partials would remain blank for MPG. You could even have a link explaining this for anytime it does a partial, or number of partials then a full tank.

You have the pricing already configured, so that part is fine.

Those using Trip readings, can still do the same thing. Just input your trips between the stops, and when you do the first fillup, Fuelly can still add the total miles and gallons and do the math. It shouldn't be hard to setup, honestly.

Whew, hope it helps, I didn't realize I was going to write a book this morning. LOL

DTMAce
Jan 12, 2010
7:39 AM

Alternatively, for option 1 that I started with, add the total amount spent for gas for both the partial fillups and the true fillup, then divide by the total gallons used during those stops. That will give you a more accurate price per gallon. Then use that when you make your entry for your price per gallon. But this is just with regards to tracking the partials yourself rather than Fuelly.

Option 2 I hope can be made possible instead. :D

DTMAce
Jan 12, 2010
7:59 AM

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!

Users of Greasemonkey and similar plugins may find this script useful: http://userscripts.org/scripts/show/66252
It parses the fillup and fillup log pages for partial fillups and calculates and displays an MPG for the first fillup after a partial fillup, with an asterisk to indicate that it is calculated.
I mostly made it for my own car and use, but maybe other people will find it useful.