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Old 09-29-2014, 07:08 AM   #1
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Average Consumption (MPG) graph

I’ve noticed in the Average Consumption (MPG) bar graph that the way it calculates the monthly MPG is based off of the average of MPGs alone, rather than a calculation of combined miles / combined gallons for the month. In a month with some small and some large fuel ups, this can give a skewed result.

For example in a month with the following:
#1 -- 400 miles -- 10.000 gal -- 40 mpg
#2 -- 200 miles -- 5.714 gal -- 35 mpg
#3 -- 200 miles -- 5.714 gal -- 35 mpg

The way the current bar graph calculates, it would show 36.66 MPG for the month. Going by actual calculation of combined miles / combined gallons for the month, the true MPG for the month is 800/21.428 = 37.334 MPG. 37.33 seems more accurate to me, but that’s only my opinion.


Also, would it be possible to display this bar graph for lifetime, similar to the ability to “Chart all fuel-ups” on the past 10 MPG line graph?

Thanks!
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Old 09-29-2014, 10:48 AM   #2
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I'm surprised they do it that way... taking an "average of averages" is pretty much a no-no in any statistical situation. It has to be weighted, or calculated directly from the miles and gallons.
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Old 09-29-2014, 12:21 PM   #3
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We don't do average of averages.

jandree22, looking at your car, comparing the September / August / July values shown in your fuelup log to the avgs shown on the Average Consumption bar graph directly above, I get the same values when doing (miles / gallons) on the fuelup log.

Could you provide examples from your log where you see an issue?
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Old 09-29-2014, 01:15 PM   #4
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Thank you freddie. From my actual records, an example from March...

1) 237.0 mi --6.656 gal -- 35.61 mpg
2) 128.9 mi -- 3.175 gal -- 40.60 mpg
3) 244.2 mi -- 6.323 gal -- 38.62 mpg
4) 256.5 mi -- 6.607 gal -- 38.82mpg
T) 866.6 mi -- 22.761 gal

866.6 mi / 22.761 gal = 38.073 mpg
(35.61+40.60+38.62+38.82) mpg / 4 = 38.412 mpg

The bar graph reports 38.4 mpg.

This isn’t a big deal, only differs by a few tenths. I noticed it accidentally earlier this month when I balanced between two Sept tanks from an overfilled tank. The bar graph curiously changed for the month even though the total miles & gallons for the month remained the same.
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Old 09-29-2014, 01:34 PM   #5
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Okay, take a look now at the graph.

You're right. It was doing an AVG of the avg mpg for each fuelup. I copied that formula straight over during the rewrite. I failed to convert that case of avg of avg, but it is good now. I had converted another instance so that is why I was sure I that wasn't going on any longer.

It is showing 38.1 for March now.
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Old 09-29-2014, 02:23 PM   #6
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Awesome, wow that was quick. Thank you, much appreciated!
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