Inaccurate mpg? - Fuelly Forums

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Old 06-27-2010, 04:07 PM   #1
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Inaccurate mpg?

Awesome idea for a site. Keep it up.

I was wondering one thing however. I started off here with a partial fuel up. 11 gals out of a 22 gal tank. However since then, I've been doing full fill-ups. However, even those are usually 20 gals out of 22 gal tank. I don't run on any less than 2 gals to avoid running out of fuel. Will that make my results inaccurate or does it matter? Thanks!
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Old 06-28-2010, 01:26 AM   #2
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Doesn't matter. The only time it makes it inaccurate is if you fill up the car, drive until it's empty, and then fill it to less than full -- that is when you should mark it "partial." If you fill the tank up as much as you can it should be accurate (as accurate as it can be at least). But if you're intentionally filling it less than full, it will not be accurate.
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Old 06-28-2010, 02:00 PM   #3
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Thank you BDC, something I was wondering about. Makes more sense to me to might as well fill it up all the way anyway since I'm gonna end up eventually paying for the tank no matter what I do, haha.

Thanks again.
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Old 06-29-2010, 02:54 AM   #4
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Yeah, the Fuelly style of calculating mileage is based on the difference from full for each tank. Without a full tank, Fuelly won't work. Here's more info: Why do I have to fill the tank all the way up every time I buy fuel?
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Old 06-30-2010, 01:53 PM   #5
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More knowledge for my noggin'! Cool. Pretty cool site I say again. Thanks!
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Old 07-02-2010, 11:25 AM   #6
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Here's an idea for partial fuel-ups. (I have this problem because my friends who borrow my truck don't fill it all the way up - and I don't expect them to!)

Add the mileage, and the fuel, to the miles and fuel on the next fill-up. Kinda like how you don't put in a bowling score when you get a strike until two more throws!
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Old 07-18-2010, 06:11 PM   #7
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I found a similar issue when I first 'filled' up. My first was a partial, which I didn't think would matter, yet it seems to have had some effect. I'm sure it doesn't matter too much, just to me when I see the average mpg when I believe it should be a little better.

Love the site! I am finding it encourages me to drive 'like an old man' to improve my economy by tracking the figures. Finally, something which puts coin in my pocket rather than taking it out with an internet site.
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Old 07-25-2010, 02:12 AM   #8
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The advantage to running the tank lower is that pumping errors (differences in how full the tank is when you fill it) are less significant the more you pump.

Ex:

12 gallon tank

Pump A shuts off at 11.9 gallons

pump B shuts off at 12.0 gallons

There's a .1 gallon difference in how "full" the tank is, resulting in a .1 gallon error in how much fuel was used. If you fill up at A, then B, you will fill .1 gallon more than you actually used. If you fill up at B, then A, you will fill .1 gallon less than you actually used.

How significant that is depends on the total amount pumped. Lets say you went 200 miles on 10 gallons pumped.

Examples:

If you actually used 10 gallons, you got 20MPG.

Nearly empty

If (because of the error in shutoff) you actually used 9.9 gallons, you actually got 20.2MPG, so your reading would be off .2MPG (not a huge deal)

If (because of the error in shutoff) you actually used 10.1 gallons, you actually got 19.8MPG, so your reading would be off .2MPG (not a huge deal)

Nearly half empty

But what if you only pumped 5 gallons and went 100 miles? Still 20MPG nominal, but

If (because of the error in shutoff) you actually used 4.9 gallons, you actually got 20.4MPG, so your reading would be off .4MPG

If (because of the error in shutoff) you actually used 5.1 gallons, you actually got 19.8MPG, so your reading would be off .4MPG

Nearly Full

In the extreme, for a vehicle that gets about 20MPG, a 1 gallon topoff could be off by 2MPG per 1/10 gallon inaccuracy!
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Old 07-25-2010, 02:21 AM   #9
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oop - the second one on nearly half empty should be 19.6MPG, but you get the idea.

The shorter the distance and fuel used, the more significant filling errors become.
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