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As long as there is some fuel in the tank, does the fuel level matter? In other words, is it advisable to make sure the tank is mostly full most of the time, or is this just silly? In other words, are there valid reasons why you should avoid running on an almost empty tank (other than the risk of actually running empty)?

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this has so many levels. great question. –  amosrivera Sep 8 '11 at 21:15
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Wow. Over 1000 views today. –  Bob Cross Sep 9 '11 at 0:43
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9 Answers

up vote 38 down vote accepted

Both previous posts are pretty good. I'll add a few more considerations though.

  • On a low tank, during hard cornering, some cars will uncover the fuel pump pickup and starve for fuel.

  • There's been some discussion for years now about keeping 1/4 tank as your minimum as the fuel provides cooling for the fuel pump. Some people argue that additional cooling will extend the life of the pump. My take on it is that indeed it does, but that it doesn't matter as the pump won't overheat either way (OEM fuel pump failures are exceedingly rare nowadays, nearly all fuel pump replacements that I've seen done turned out to not solve the problem they were having...).

  • Any floating debris will stay safely above the pump intake if the tank level is kept up. Run it down far enough and that stuff will get sucked in and contribute to either clogging the filter eventually, or even physical damage.

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Debris is a winning point: there will be gunk in the tank and a super low level in the tank increases the probability of injesting that gook significantly. –  Bob Cross Sep 8 '11 at 19:58
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Side point on the "keep a minimum of 1/4 tank": it's actually handy for spouse cooling as well. Ever been late to an appointment with your loving significant other and had to explain that you're nearly out of gas...? –  Bob Cross Sep 8 '11 at 20:00
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It should be noted that any "floating debris" are not going to be prevented from entering the pump simply because you have some gas in the tank. The first time you turn a corner or hit a bump, the tank will be thoroughly mixed, giving even floating debris a chance at the pump intake. –  Fake Name Sep 9 '11 at 3:35
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@Fake Name, actually you won't be mixed in a single turn. Mixing will occur all of the time. The volume in the tank reduces the density of the gunk in terms of stuff per unit fuel volume. You are correct that there isn't a magic spell, fuel or not, that will prevent all injestion ever. –  Bob Cross Sep 9 '11 at 19:45
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Water is heavier than gas and will stay at the bottom of the tank. True, after a little driving the water will mix somewhat with the gas so your going to get a little water sucked up anyway. I like to run as low as I dare so as to get as much water out as posible. A small amount of water running through the engine won't hurt, in fact in some cases a little water is good.(ever hear of water injection) Debrie I don't worry about but I do worry about the extra water produced by the use of ethanol. Best to run this water out a little at a time rather than to let it sit and destroy your tank.

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Wow ... Cannot count on how many levels running your fuel tank empty is bad. There are other ways to get water out of you tank than letting the engine suck it up. If you have that much water in your tank you will stop your engine dead when it sucks it up. If you have an older vehicle and run it dry, you will probably clog the in-tank filter with any sediments. A lot of cars use the gas itself to cool the pump. When you run it dry, you can overheat your pump, shortening the life span of the pump. Just really is a bad idea. Try to keep the tank at least half full to avoid problems. –  Paulster2 Feb 3 at 13:56
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He is correct! The actual pump is inside a canister and unless the tank is bone dry, the pump is ALWAYS submersed in fuel ( even when cornering hard). if this was not the case, the vehicle would stutter and hiccup due to air in the line. Anyone who has ever had this happen, knows it does this only when you completely run out of gas. Just look at the design for yourself and use common sense instead of listening to opinion and hearsay.

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it doesn't matter how low your fuel level is, the fuel pump will always be submerged in fuel, the pump actually sits inside a whell that is full of fuel, its like this on every car that has an internally mounted pump, otherwise the mass would be exposed to oxygen and a spark can ignite the tank vapors and boom. --- former UTI graduate.

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As Brian's top answer says, it is on cornering that this becomes an issue. –  Rory Alsop Jul 10 '12 at 12:20
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If you are planning to let your car sit for a while, it also may be adavisable to have a full tank. Less air in the tank means less corrosion of the tank, if you have a metal one.

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Even if you have a plastic tank, water in the gas is not good. –  Mark Johnson Apr 12 '12 at 5:08
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But if you are planning to let your car sit for a LONG while, I'd drain the tank completely. –  Jan Steinman Oct 2 '12 at 18:24
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I always let my fuel level reach near empty. Other than the cool effect on the pump, I see no advantage of not letting it get below 1/4 tank. Water is heavier than gas, so if there is any water in the tank it will be picked up by the pump first. Same goes for any debris in the tank. The pump is also situated so that on cornering the fuel pump will not be deprived. I will admit that I have run out of gas a number of times, not for a few years now, so feel free to discount my comments.

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If you ever drop the tank, see if the pump wiring harness is a bit crispy. –  Mark Johnson Apr 12 '12 at 5:09
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I also would add that, besides avoiding problems with running always with less than 1/4 (probably, a few times would be alright), you could also avoid being without gas if you are in a jam or need to go to a place which is far than you expect, like giving a ride, and you don't have time to fill the tank or there is no gas station near.

Anyway, I always prefer to be safe than sorry. And I guess there are few reasons, other than monetary, not to keep it with at least 1/4 filled.

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Monetary? Since the price of gas is constantly increasing, it's better monetarily to fill it up now while it's cheaper. –  endolith Sep 12 '11 at 0:54
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@endolith I really see no difference, since really quick you'll have to pay the new price. Unless is up to 10-20% a week. And also, you're contextualizing that question to US residents and also to an specific period of time where the gas is rising a lot, which might me biased and not generic enough to this question. –  lucasarruda Sep 14 '11 at 21:40
    
The "monetary" issue is an interesting one. Consider that if everyone keeps a full tank rather than a ¼ full tank, the petroleum industry gets four times as much money sooner -- and they don't have to build four times as much storage! So help the oil barons out and fill your tank! :-) –  Jan Steinman Oct 2 '12 at 18:22
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Nothing like going to a station with your car on "E" and finding that the gas door release has broken and there's no way to get gas in the car. :-) –  Brian Knoblauch Feb 5 at 17:30
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There is an unexpected effect to running with a full tank all of the time: fuel is heavy. The actual weight per gallon depends on temperature (and thus on density) but it's on the order of six pounds per gallon (or about .7 kg per liter, if you like doing all your math in base 10...).

Obviously, the total weight depends on your vehicle's tank. In my car, driving full all of the time would be an additional 90 pounds. That has a measurable effect on mileage, especially in the in-town lowish speed driving that I'm often doing.

I generally fill up, run it down to about 1/4 tank, fill and repeat. On average, I see better mileage than "full all the time."

As Gabriel notes, however, weather can trump mileage.

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The weight of the gas depends on weather, but the mass of the gas is constant. And the drag on mileage is caused by mass, not weight. –  corsiKa Jan 2 at 16:17
    
@corsiKlauseHoHoHo, if you'd like to have a open-ended discussion about the physics about the multivariable system that is a car, please do that in the chat room. –  Bob Cross Jan 3 at 1:56
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Knowing that gasoline does not freeze (Ok, it will start freezing at -180 degrees, but this is more than you or your car can tolerate) but that a small amount of water can accumulate in yout tank over time.

So, in cold winter weather condition, like here in Canada, if you have a low level in your tank, that accumulated water could freeze and cause some issues, like blocking your fuel pump or hoses.

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You're right: condensation in the tank can be a hassle well before freezing, especially if you're running your tank at a low level all of the time. –  Bob Cross Sep 8 '11 at 12:10
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I always store my garage queen with a full tank for this very reason. If you've got a metal tank, it's even more important. Corrosion in a fuel tank sucks. –  Mark Johnson Apr 12 '12 at 5:06
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Gasoline actually freezes (or a firm facsimile there of) at around -40 to -50C according to this source. Check the follow-up question at the bottom of the page. There are a lot of opinions out there, but believe a physics page might be correct. –  Paulster2 Feb 3 at 22:12
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