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jrferrari

All-Star Author
Orlando

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Message Posted: Oct 29, 2012 12:03:57 PM

I've tracked my mileage for a very long time. I started with a paper log book and progressed to an excel spreadsheet where I created graphs from the data (price over time, mpg over time, cost per mile, cost per day, even seasonal mpg were I group by mmm/dd across multiple years, etc.). I started entering my tank fill information into GB and it seems that it might be calculating incorrectly.

If I decide to start entering my fill information into GB for the first time I must fill my tank and Missed Previous Fill must be Yes. The only relevant information on this record is the odometer reading (Gallons Purchased is not relevant). The mileage can not be calculated with just this one record.

If the next record entered is Filled Tank = Yes and Missed Previous Fill = No the MPG can be calculated, this was not my experience. If you take the odometer reading from the 2nd record entered and subtract the odometer reading from the previous record you can calculate the distance travelled in miles, divide this by the gallons in the 2nd record to calculate the MPG. If I enter two records in this sequence the MPG is not calculated. When I enter a third record the MPG is calculated for the second record, but it is calculated incorrectly.

If you look at my records the mileage on the second record should be 25.25 (210 miles / 8.316 gal) but GB calculates 25.60. GB should also be able to calculate the mileage for the 3rd record but doesn't. GB is collecting all the information that is relevant, but I don't think you are calculating correctly.

I haven't tested it yet, but if you have a Filled Tank = Yes record followed by one or more Filled Tank = No record, when the next Filled Tank = Yes record is entered you should be able to calculate the MPG for that record. Calculate the mileage between the two Filled Tank = Yes records and aggregate the Gallons Purchased for the Filled Tank = No records and the current Filled Tank = Yes record, then do the math.

Hopefully this can help fix the feature.
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Don
Moderator
Message Posted: Nov 15, 2012 10:40:43 AM

@ GLM4205,

Thanks, yes we do like to take some time to interact with the GB community, it's what the "Talk back to us!" forum was made for. Outside of responding to large volumes of feedback and performing other site related tasks, the GBMods are here to help (when time allows of course)!

-Don
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GLM4205
Champion Author Toledo

Posts:6,486
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Message Posted: Nov 15, 2012 7:26:52 AM

GB must have lot of time to do in this busy schedule.
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Don
Moderator
Message Posted: Nov 1, 2012 3:41:50 PM

You need to have Missed Previous Fill set to NO for the calculation to have a starting point, think of "Missed Previous Fill" as "Missed Previous Entry".

The calculator needs to have two complete fill ups provided back to back in order to calculate your MPG.

By default, if this option is set to YES it will start the log over, regardless of whether its the first fill or not.

If it's your first entry, you should have this set to NO (but also ensure, just like the calculator says, that the fill is a complete fill up, which your first one does not appear to be).

This is put in place because you should not be using odometer readings that may make it seem like you've traveled further than you really have on one tank of gas (ex. make it look like you've traveled 500 miles on one fill, when really you traveled 250 miles on two separate fill ups, this is an important thing to differentiate).

In your case, you nullified your first entry by setting your Miss Previous Fill on 10/14/2012 to YES - your first MPG would have then been calculated by your second and third entries.

When you have the initial fill set up correctly (ie. Miss Previous Fill set to NO), you will see that the MPG for your first entry is in fact 25.25 MPG (as you previously described that the figure should be).

I think you may be looking at thing things a little backwards, 25.60 MPG reflects the amount of fuel purchased on 10/21/2012 and uses the distance traveled by your next entry on 10/27/2012, not the amount of fuel purchased on 10/14/2012 and the distance traveled between then and 10/21/2012 (which would be 25.25 MPG if you had Missed Previous Fill set to NO).

You need to be able to record the distance that a (full) tank of gas has taken you. You need to be able to record the amount of fuel that you purchased, and the distance that amount of fuel allowed you to travel (obviously this will not always be the same for every fill when factoring in daily stop and go traffic, weekend trips, unnecessary idling, etc but all the calculator cares about is the distance you traveled and how much fuel you used, it does not take into account that your AC was on or that you changed your spark plugs etc, feel free to note that with your fill in the Notes section).

With this in mind, your odometer reading (the distance traveled) can only really be provided after "driving off" that tank of gas.

After you have the distance traveled on the tank of gas, and the amount of fuel that your purchased and used, MPG is calculated using that distance traveled by the amount of fuel used, Distance(miles driven, odometer reading)/(per or divided by)fuel used(gallons).I'm not entirely sure how MPG could be calculated any differently.

I hope this explanation helps.

-Don



[Edited by: Don at 11/1/2012 3:58:07 PM EST]
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jrferrari
All-Star Author Orlando

Posts:652
Points:172,990
Joined:Jan 2011
Message Posted: Oct 31, 2012 10:37:24 AM

Actually when I record my mileage in a spreadsheet I calculate it both ways.

I calculate it the "normal" way where the MPG is associated with the most recent record so I can correlate it to conditions during the time when the gas was used (city/highway, use of A/C, towing, hard acceleration, etc.).

I calculate it the GB way to associate the MPG with the fuel to correlate it with the brand of gas, grade of gas, station location, even the pump (if it measures gallons incorrectly).

I see value in doing it both ways, but I think most people would want to see it the way fuelly does it.
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reb4
Champion Author Chicago

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Message Posted: Oct 30, 2012 3:10:27 PM

Jrferrari,

I believe the intent is to correlate mpg to the actual gas you used. (remember I'm explaining it, not defending it). When you put the fuel in the

I have kept mpg for several vehicles and so it's kind of a thing I got used to using... I have put one of my vehicles on fuelly and it is fun to see the graphs and different statistics. However I still also keep info and actually put the fillup in the logbook 1st.

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cookieguru
Sophomore Author Seattle

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Message Posted: Oct 30, 2012 1:09:00 PM

Not everyone thinks like you.
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jrferrari
All-Star Author Orlando

Posts:652
Points:172,990
Joined:Jan 2011
Message Posted: Oct 30, 2012 10:06:02 AM

I've used fueleconomy.gov in the past which has similar functionality to Fuelly without the fancy graphs. It calculated tank mpg as expected too.

I guess my point is why does GB calculate it differently? It should calculate tank MPG as expected.
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cookieguru
Sophomore Author Seattle

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Message Posted: Oct 30, 2012 4:01:30 AM

This is because GasBuddy thinks of a tank mileage differently than you do. It's on the row prior because you don't actually know the MPG for the gas in your tank until you top it off.
For example, when you buy a new car, you get a full tank of gas. There's no MPG to compute since the car hasn't been driven (much). On your first fillup, you can calculate the MPG for that tank. And that's how GB displays it, even though you and I think it should be displayed with your most recent fill up.

Just sign up for Fuelly and skip using the log book here. The graphs are much prettier, the UI is much better, and the MPG to tank calculation works as you'd expect it to. It also allows you to compare your MPG to other vehicles of the same year/make/model/class and gives you info on how to save more (e.g. get +1 MPG and save $0.20 on your next fill up).
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