Why does Fuelly skip calculating MPG on the fill-up after a partial fill-up?
It seems to me, that the next fill-up after the partial fill-up could sum together its own data with directly previous partial fill-ups to get an accurate MPG calculation for the entire period.
Is there a reason why the next fill-up is skipped?
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.
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.
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.
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.
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:
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
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.