MPG calc incorrect - Fuelly Forums

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Old 03-18-2013, 02:57 AM   #1
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MPG calc incorrect

Accuracy of the MPG calculation seems to differ from the basic maths?

Why would my avg UK MPG figure be showing 42.4 when the total miles are 25511, litres are 2770.11 and calculating a UK mpg which actually equals 41.87 for my 123d?

Also found the same on my Kuga.
Total miles of 14588, litres are 1977.91, shows avg mpg to be 34.5 yet the correct calc is 33.53?
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Old 03-18-2013, 03:50 AM   #2
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Because you are using odometer rather than trip tanks, it's the 33.85 litres from your very first fuelling. It is not included in the calculation because it didn't know the odo reading at the start of the tank (i.e., it knows miles from the end of that first tank, to the end of the most recent tank, so it only look at the fuel used between those points).
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Old 03-18-2013, 04:16 AM   #3
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Thanks ;-)

So what's the best way to correct it?
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Old 03-18-2013, 04:43 AM   #4
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The 42.4mpg calculation is correct, but to make the figures make more sense, you could edit the first fuelling so that it says zero litres.

Alternatively, if you know the miles driven for the first tank, you could insert a new fuelling prior to the first (again, with zero fuel).

Either would mean that the total fuel and the total miles recorded in fuelly are limited to the significant readings only (currently, the 'total fuel' is misleading because it includes the fuel from the first tank which cannot take part in the mpg calculation).
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Old 03-18-2013, 08:42 AM   #5
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Thanks, will edit the previous ones and sort out for my new car which I hopefully get on Thursday ;-)
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Old 03-19-2013, 02:46 PM   #6
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MMUK, if that's the case, shouldn't it be part of Fuelly's calculation for MPG to ignore the volume of the first fill up for vehicles that use odometer? I use odometer tracking on both of mine because my scooter doesn't have a tripometer, and when my wife gets gas she often forgets to reset the tripometer in her car.

You say the 42.4 mpg is correct, but it isn't. It's using X number of fillups for the volume, but X-1 fillups for the distance. Sounds like a calculation error on Fuelly's end to me.

Thanks for pointing this out lanh, I'm going to edit my first fill-ups to be 0 gallons.
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Old 03-20-2013, 03:37 AM   #7
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>... shouldn't it be part of Fuelly's calculation for MPG to ignore the volume of the first fill up for vehicles that use odometer? ...

Exactly, that's precisely what it's doing.

> ... It's using X number of fillups for the volume, but X-1

>fillups for the distance. ...

Nope. When it is calculating the MPG, it is using X-1 for both. Hence you get the correct 42.4mpg.

However, when it is showing you total distance and total fuel volume... it is showing you X-1 for total distance, but X for total fuel volume. That's why when are you trying to calculate it manually, you are getting the wrong number.
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Old 03-20-2013, 03:42 AM   #8
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>Thanks for pointing this out lanh, I'm going to edit my first fill-ups

>to be 0 gallons

You will find that fuelly's MPG figure is not changed(because it has already excluded it), but the 'total fuel volume' figure will become consistent with the MPG calculation.
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Old 03-21-2013, 11:04 PM   #9
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Ok, I see what you're saying MMUK. If you manually calculate it using the total gallons and total distance fuelly displays, you wont get the same number fuelly shows as your average MPG because fuelly is completely ignoring the first fill-up, but it's still taking that first fuel-up into account for things like total gallons, price, and number of fill-ups.

Makes sense to me now
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Old 03-22-2013, 10:59 AM   #10
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>Makes sense to me now

Excellent :-)

It took me a while to figure out the problem Ian had run into, because I use the trip-meter method instead of the odo method. When using the trip meter method, the fuelly totals for fuel & miles are consistent with the mpg. (I don't think it's possible to swap between the two methods).
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