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Old 09-30-2008, 04:47 PM   #1 (permalink)
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2 Tank Averages - Anybody else use them?

Hey all,

I seem to be having the ups and downs in my gas logs lately. I decided to try using 2 tank averages. Take the first tank miles and gallons and add the second tank miles and gallons. Divide total miles by total gallons and you have a 2 tank average. Next time add the 2nd tank figures to the 3rd tank figures and so on. This gives me a much smoother view of my progress as it evens out the uneven fills. Here is my chart with the normal and 2 tank figures. Anybody else do this?


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Old 09-30-2008, 05:51 PM   #2 (permalink)
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The EM fuel log has like 90 day and 3 tank averages which I find useful, but I usually don't pay attention to much besides what the mpguino is telling me.
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Old 09-30-2008, 09:01 PM   #3 (permalink)
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I think it's just normal to get the high's and low's along the way because of weather, traffic, and all the other variables. I always top off the tank and even doing that get some pretty large variations.
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Old 10-01-2008, 01:40 PM   #4 (permalink)
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Quote:
Originally Posted by Ford Man View Post
I think it's just normal to get the high's and low's along the way because of weather, traffic, and all the other variables. I always top off the tank and even doing that get some pretty large variations.
Thats usually how mine are, since many tanks are similar conditions but rarely are 2 adjacent tanks similar conditions. The only time I consider the average of adjacent tanks is when I get a bad fill (like when shutoffs are broken on pumps and it overfills). Then Ill consider what I lost in mileage with my apparent gain on the next tank. I do try to fill each tank as consistently as possible, filling at about .1gal/sec and then going to the 2nd kickoff.

I check every tank against the guino. From my initial settings based on the injector flow rate, it reads about 4% high +/- 4% on fuel use, which is still pretty dang close. It could be due to fill variations (and pump calibration). It could also be because of the fuel filter I need to put in once the weather shapes up. Im not changing any guino settings till the new filter is in for a few tanks. However one thing I like about the current setting is that I know Im at least getting what the guino says, Im never disappointed when I calculate the actual fuel use.
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Old 10-01-2008, 04:37 PM   #5 (permalink)
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I tend to focus on the tank averages thenselves - but for plotting i am tracking 3 tank averages in addition to the actuals
- although 2 might be better as i would be spanning a 2 month period than a 3 month one -
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Old 10-02-2008, 03:16 AM   #6 (permalink)
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My tanks seem to not vary a whole lot, but they are so long, that it may as well be two tanks.
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Old 10-02-2008, 03:39 AM   #7 (permalink)
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moorecomp -

I wouldn't worry about the tank-to-tank. Watch for the overall trends in the data. Your movin' up from what I can see, .

Also, because your fuel log started in the summer, you should expect it to go down because of your winter weather in Michigan, .

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Old 10-02-2008, 06:24 AM   #8 (permalink)
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Quote:
Originally Posted by SVOboy View Post
The EM fuel log has like 90 day and 3 tank averages which I find useful...
What do those 'black dots' on the 'EM fuel log' graph signify?!?!?

Assuming they are lifetime averages - I would say this is the way to go...
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Old 10-02-2008, 08:38 AM   #9 (permalink)
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This is a normal statistical relationship. But it is all really an artifact because you still have to fill up the tank every time it is empty. All the variability that is introduced at that time is still there whether you record the MPG then or not. In order to get more precise MPG data you would have to actually double the size of your tank, not just skip a data point between two tanks. It is best to take the measurements for every tank because you then have the most versatility with your data.

How many tanks you should average in your spreadsheet depends on what you want from your data. If you want the best absolute estimation of your car's mileage, average all your data points and have one average MPG figure (but that's a very boring chart ). If you are interested in seeing how your mileage changes over time in response to changes in driving habit, season, or aero devices you should pick shorter measurement periods (per tank for example) and put up with somewhat higher variability from the true average. If you are interested in how your mileage is trending just have your spreadsheet calculate a trend line using individual tank measurements.


Last edited by instarx; 10-02-2008 at 09:10 AM..
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