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Do you always fill fuel to full?
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Thread: Do you always fill fuel to full?

  1. #1
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    Question Do you always fill fuel to full?

    Hello,

    Some said filling up full all the time will get lower fuel economy due to no space for evaporation (EVAP systems) to work?

    And also car is heavier with full tank and engine has to work harder, so that we get lower fuel economy? Are they true?

    Just wondering if you fill up to full all the time? At which point do you fill up the fuel? (example: half, almost empty)?

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  3. #2
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    Yes, I believe the difference would be negligible. More importantly, I still have PTSD from sitting in the backseat of my father's car seeing the low fuel light always on. He would put 3 gallons in at a time. LOL

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  5. #3
    Riding Lawn Mower
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    What you considered is true. In most cases, the fuel cost when you drive once more to the gas station is similar to the cost you saved, so there's not much difference.

    The reason I would do full tank is to get better vehicle weight balance. I also throw a toolbox in my trunk. This makes my car more balanced.

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  7. #4
    Riding Lawn Mower
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    File this under the same category as the "Americans waste billions of gallons of gas because we're fat".

    Gas weighs 6.3 pounds per gallon, so not putting in that last 5 gallons "to not fill your tank" only saves you 31.5 pounds of initial vehicle weight.

    Worrying about that would be like saying you don't want to do all the grocery shopping at one time because the groceries would weigh too much and affect your gas mileage, but you end up going to the store 3 or 4 extra times, wasting gas with extra trips and start/stops.

    As far as the EVAP system is concerned, the ideal situation would be to have a full tank. The EVAP system is a passive emission control and only works as needed to collect gasoline fumes and send them to the engine when they accumulate in the canister.

    Here is where a little bit of physics kicks in to have a full understanding. Air, by volume, can only hold a certain amount of a particle in suspension. Take for instance water, if this rule were not true all the water in the world would have been continually evaporating over time and never returning to its liquid state. When we say that the humidity is 75%, we mean that the air is holding 75% of its capacity of water vapor. To quantify this (these numbers are not the real numbers), if a cubic foot of air can hold 4 ounces of water, then 75% humidity means that there are 3 ounces of water per cubic foot.

    Now take that example to your gas tank. The gas tank is a sealed system, and when you have a full tank there is a very small volume of air in the tank, thus your evaporation is limited because air can only hold so much suspended gasoline per cubic foot. When you tank is at half, you have increased the air volume, thus allowing more evaporation.

    Here the EVAP system kicks in. Besides proving a controlled vent for the gas tank (to avoid negative pressure when gas is used), it takes the fumes coming from the tank and passes them through a charcoal canister and then to the engine to be burned.

    As far as when I fill up, I do it every 2 commute cycles. I could (and have done) go three commute cycles but that takes me down too far into my reserve for major traffic issues. I always top it off when adding fuel.

  8. #5
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    I find it cheaper to fill up. Besides, it's so annoying always having to look at your gauge to make sure you have enough for your destination.

  9. #6
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    One thing that is true, at least on certain cars, is to NOT continually click the fuel nozzle to try to cram extra gas in (top-off). I see people do this, and on some cars at least, this can cause liquid fuel to enter the evap system, and cause it to fail.
    It is best to fill up till the first click off, and that's it, do not try to "top-off".

  10. #7
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    Quote Originally Posted by rpoffen View Post
    One thing that is true, at least on certain cars, is to NOT continually click the fuel nozzle to try to cram extra gas in (top-off). I see people do this, and on some cars at least, this can cause liquid fuel to enter the evap system, and cause it to fail.
    It is best to fill up till the first click off, and that's it, do not try to "top-off".
    Guilty. After the first click, I manage to put an extra 2-3 liters.i guess I'll stop doing that.

  11. #8
    Riding Lawn Mower ThatsNickey's Avatar
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    Quote Originally Posted by 1st timer View Post
    Guilty. After the first click, I manage to put an extra 2-3 liters.i guess I'll stop doing that.
    Likewise. Used to filling it up and going one or two more clicks. One click it is from now on.


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  12. #9
    Riding Lawn Mower NBimprt27's Avatar
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    I always filled to full. Simply for my personal mental aspect that I drive more fuel efficient when I just filled it up. Filled up when I'm at about 1/4 tank.

  13. #10
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    Roughly 50% of the time I typically wait until the fuel gauge hits the 1/4th full mark before refilling, 45% of the time when it's at 1/8 full, and 5% when the low fuel light comes on (yeah I know ideally I shouldn't wait until then but sometimes things don't work out timing-wise but even then when refilling at the low fuel light mark I find that there is still roughly 2 gallons of gas remaining in the fuel tank).
    Last edited by tonester; 04-25-2019 at 02:16 PM.

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