Working As Designed: MPG numbers don't match [non-bug] - Fuelly Forums

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Old 06-17-2014, 09:22 PM   #1
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MPG numbers don't match [non-bug]

Don't know if this is a feature or a bug.

Dashboard:



Car detail:
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Old 06-17-2014, 09:59 PM   #2
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Thanks for the screenshots. Your dashboard MPG or "Running Average" may differ slightly as it's only taking into account the last 10 fill-ups.
More info can be found in our FAQ: http://www.fuelly.com/faq/35/running-average
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Old 06-22-2014, 08:35 PM   #3
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Hi Fuelly Admin
To avoid any confusion. Is it possible to have the "running avg mpg" on the dashboard changed to "avg mpg"?
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Old 06-24-2014, 11:51 AM   #4
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Quote:
Originally Posted by giant9765 View Post
Hi Fuelly Admin
To avoid any confusion. Is it possible to have the "running avg mpg" on the dashboard changed to "avg mpg"?
Cheers
May I ask Why?
From our FAQ:
Quote:
Your all time average doesn't change very much over the life of your vehicle. Looking at the running average gives you a quick snapshot of your most recent fuel economy.
Just curious what your reasoning is to justify the change so we may consider it.
Thanks.
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Old 06-24-2014, 12:18 PM   #5
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May I suggest calling it " Recent Average MPG". I think that does a better job in conveying the number shown is less than all entries.
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Old 06-25-2014, 04:24 AM   #6
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Originally Posted by Member2014 View Post
May I suggest calling it " Recent Average MPG". I think that does a better job in conveying the number shown is less than all entries.
I like this suggestion
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Old 06-25-2014, 08:40 AM   #7
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What about adding the "Running Avg" as something we can add to the custom mini stats on the page. Because currently I have to leave the detail page and go to the dashboard to see it.

And I like having that, because as you said, I can compare recent mileage to my all time average to see if I'm doing better in my driving habits.
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Old 06-25-2014, 11:21 AM   #8
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Avg MPG

Hadn't even noticed the difference between those two figures, as I rarely see/use the Dashboard feature.

My bigger gripe, which I addressed with Fuelly in an email conversation with "Robert V" on 2/12/14, is the method used to calculate Avg MPG. It appears that the calculation routine averages the MPG figures from each fill-up to arrive at what they call "Avg. MPG." However, this arrives at a fictitious number, or at least a false impression, depending on how one interprets the expression, "Avg. MPG."

As I pointed out in February, averaging the MPG figures calculated from each tank is averaging a list of averages. The more layers of averaging, the more distortion of the actual picture. When averaging the averages, the reported figure will be skewed by how often one fills up -- i.e. on "E" or at 3/4, 1/2, etc. (and more importantly, how consistently one fills up) -- as well as the kind of driving which took place during each fill-up. The error will be more pronounced, the less consistency there is in one's driving demands.

In other words, if each fill-up is not fairly close to the same figure (perhaps due to changing demands, such as long business or vacation trips vs. stop-and-go city driving), then the "Avg. MPG" figure will be skewed in favor of whichever style of driving is the most common/frequent, rather than presenting an accurate picture of the vehicle's real average efficiency. This is because the present calculation is averaging the results from fill-ups not true miles vs. gallons. The better method, as I also pointed out in that conversation on 2/12/14, would be to calculate TOTAL miles driven per TOTAL gallons consumed. That would be a true average MPG over the life of the vehicle.

Just as a specific example, one of my vehicles currently is reported by Fuelly as having an Avg. MPG of 13.4. In fact, when TOTAL miles are divided by TOTAL gallons, the true average MPG over the life of the vehicle is calculated at 14.1. I know, what's 0.7 MPG among friends, right? In this case, it's 5% error. The point is, if this app is to be useful, the figures must be accurate; if they are not accurate, then they are meaningless and useless at best.

I had hoped this would have been addressed in the recent update (as Robert V. suggested it might), but it evidently was not, unfortunately. It would be great to see Fuelly correct this glaring flaw in an otherwise very good app.
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Old 06-25-2014, 11:58 AM   #9
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The avg numbers, for any period, of any stats, are being generated via a miles / gallons formula. When we generate these averages we ignore all fuelups that don't have a MPG or don't have a miles value stored. A fuelup might not have a MPG for several reasons, you marked it partial, you marked the previous partial, you put in an excessive number of miles, say 5,000.

So when I ask the database to tell me your numbers, I get 18,200.20 miles and 1,351.437 gallons. If you figure that out, you will get 13.36 which is where the 13.4 average comes from.

What numbers did you use to arrive at 14.1?

The numbers shown in the "Total Miles Tracked" and "Total Gallons Fueled" mini stats will differ from these numbers as those are displaying the total distance and miles, regardless of the mpg status of the fuelups.

These are SUM(miles_last_fuelup) and SUM(amount) versus the formula used for averages, which is along the lines of SUM(miles_last_fuelup) and SUM(amount) WHERE mpg > 0 and miles_last_fuelup > 0

The only number that is an avg of avgs is the Running AVG displayed on the dashboard. I need to change the data structure of the stored last 10 fuelups to resolve that one.

See my test car: http://www.fuelly.com/car/acura/mdx/2009/freddie/298667

It illustrates what I am saying.

Miles / Gallons = 19.8 MPG
Sum of Avgs / 4 = 17.3 MPG
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Old 06-25-2014, 12:37 PM   #10
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Oops!

Quote:
Originally Posted by freddie View Post
What numbers did you use to arrive at 14.1?
Well, Freddie, unfortunately it looks like I'm going to have to eat some crow here. The numbers you used, do not match mine, but in going back to my spreadsheet to find out why, I discovered an oddity in the way Apple Numbers handles a particular formula, so it turns out that my spreadsheet was giving an unexpected erroneous figure.

I had expected (obviously wrongly!) that the best way to calculate total miles/total gallons would be as follows: =sum ((miles column)/(gallons column)). This would be accessing actually entered data, rather than the results of a calculation as would be the case if I summed each column and then divided the sums.

The miles column totals 19006.2 currently, and the gallons column totals 1413.45. Much to my chagrin, 19006.2/1413.45=13.446673! :scratches head: So I go and change the formula in the Average MPG field to =(miles sum cell)/(gallons sum cell), and voila! My average MPG now matches yours. Hopefully egg is good for the complexion.

APOLOGIES FOR HAVING MALIGNED THE APP UNJUSTLY.

Edit: P.S. It is also good to know that the calculations are actually being performed by the more accurate method now, as Freddie's test car proves. In our previous conversation, Robert V. stated, "The Average MPG figure is calculated from an average of each individual fuel-up MPG reading." This is the info I was basing my arguments upon.
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