Is it really a good idea to always keep the gas tank full?

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On a lot of websites many People say it’s a good idea to fill up the gas tank when it gets to be half full. A lot of them do so so they can get out of Dodge in a hurry.

I can see their point, however; if a Person does that all the time don’t they wind up with a half a tank of bad gas every month, all year long?

Isn’t that a bad thing?

Unless of course you use fuel stabilizer. But who does that on an everyday basis?

I just wondered about that. How bad is it to do so? Is it a real minor issue or does it add up to huge maintenance and repair bills much sooner than if you waited until the tank was under 1/4 full to fill up? Or is it nothing?

I know it used to be important to keep the tank full in the Winter in older cars, is that still true? Shh, I’ve been doing it, but I’ve forgotten why. Ha.

I think I read somewhere that on some cars if you let the gas level get too low you can damage the electronic fuel pump, i.e. long after the little fuel light comes on on the dashboard. As I scratch my head to remmeber, I think that happened to me once, or maybe it was from running it out of gas completely? I forget which. Modern fuel delivery systems seem so delicate to me.

In case you didn’t know, while using Firefox, I can only see the first five lines of my question in the Post Comment box. If I want to say more I have to copy and paste it from Notepad. Or maybe this is how you encourage People to keep the questions short? People like myself do tend to go long, so I could understand that.


  1. I always keep my gas tank full..I always refill just before it gets to half tank. I have been notice a strong gas smell recently when I refill. Does this means I should let my gas tank go almost empty before I refill. Does the strong smell represent stale- old gas.?

    • Hi Tasia,

      All modern cars – those built since at least the 1980s – have sealed fuel systems, for emissions (vapor emissions) control reasons. When the gas cap is screwed on, no fuel- or vapor – should be able to escape. The fact that you smell gas indicates a leak. With gas this is obviously more than just an emissions issue. I’d have the car checked out right away.

  2. The weight of the “extra” gas is insignficant. There’s better way to increase fuel efficiency.
    The gas in the tank gets well-mixed with driving. If you’re filling up frequently, you shouldn’t have any “bad gas” issues unless there’s a contamination problem. I remember the old story about a car that kept stalling due to water in the fuel system. Turns out the neighbor’s four-year old boy was playing “filling station attendant” with a hose from the faucet! A locking gas cap or lockable cap cover works wonders!

  3. You need to keep at least 1/4 tank for your fuel pumps sake if for no other reason. And many of the above reasons given are very good ones.

    Modern fuel pumps are dumping about 120 watts of heat (Yes they run at 8-10 amps) into the fuel when they are running, since port injection requires 35-45 psi (220-300 kPa) to operate in spec. I suspect direct injection will require more power.

    Running out of gas is a death sentence to the the electric fuel pump as it uses the gasoline to cool itself and lubricate itself. The modules (basically a 2-quart reservoir with a jet siphon) do help to keep the pump submerged as they siphon the last bit of fuel off of the bottom of the tank, yet they can only help.

    And nothing turns a vehicle into an over-sized paper weight like a dead fuel pump.

  4. After my wife who likes to keep the reading over half full had run out on the freeway at 1/3rd full and was stuck waiting in 105 degree for R Assist, who could not diagnose the fault and assigned the thing, via flattop, to the workshop. My mate (an aircraft electronics tech)later revealed that if water gets into the new digital tank sender units on modern cars from watery fuel, pumped in that that will screw up the accuracy of the reading big time. Ricodemus

  5. SojournerMoon wrote, “It takes several months for gas to “go bad,””

    I’ve read and heard it only takes thirty days, or less.

    You’re the first Person I’ve come across which has said it takes several months. Source? Experience?

    • It depends on the gas (is it 100 percent gas? or 10 percent ethanol?) and also storage conditions. Real gas, treated with stabilizer, stored in an airtight container in a fairly constant temperature area, can be stored for several months without problems. I’ve done it – and do it, every year. The key is to use real gas, use Stabil, use a good container – and keep the fuel out of direct sunlight or extremes of temperature.

      Conversely: If you start with untreated E10 (which is what most unleaded “gas” is), store it in a jug that’s not sealed – and leave it in a barn – it’ll go bad pretty quickly.

      PS: Try to buy your fuel from a high traffic station. This way, you’re likely to start out with fresher gas that will also last longer – all else being equal.

  6. I agree with Eric. I fill up when I hit 1/2 tank for both functional reasons as well as get out of Dodge reasons. I do not use fuel stabilizer.

    – Over time, the gas tank accumulates flotsam in it in the form of trash, flakes of crud, etc. that swirls around. This is the main reason you have a fuel filter (sometimes two) in the fuel line and why they need to be replaced. Now imagine that you have the same amount of crud sloshing around in half the volume of fuel. In effect, it makes it that much more likely that a nice large bolus of crud will plug up the fuel filter.
    – Condensation is the problem of water condensing out of the air on the inside of the gas tank. This adds water to the fuel, which is not all that good to begin with, and can cause other problems (rust) if it accumulates too much. The smaller the volume of air in the tank, the better.
    – Oxidation is another problem where the hydrocarbons in the fuel combine with oxygen in the air and make the gas bad. The less air it is exposed to, the less oxidation.

    Get out of Dodge
    – Obviously you will have a longer range in case of emergency if you have a full or nearly full tank of gas. It also saves you a stop on the way out of town. That’s the obvious benefit. But there’s more to it than this.
    – If there is a local/regional emergency, one of the first things people tend to do is gas up. Therefore there are almost always long lines at the pump, lots of angry people, and a major delay to your ability to do any other prepping (packing a suitcase, etc.) before leaving.
    – Also, in cases of local emergency, I have seen gas stations literally run out of gas on many occasions.

    Lest you get the idea that I am purely talking to the preppers who are worried about TEOTWAWKI (the end of the world as we know it), this applies equally to everyone else. I lived in hurricane country for about 4 years and had to evacuate 3x in that period. Having the luck to have a nearly full tank of gas meant that I cruised right past the long lines at the gas stations near predicted landfall and didn’t have to fill up until I was well out of harms way. The further out I got, the more gas was available. None was available in town when I left. There was an interesting market effect I noticed of some mid-way stations being out of all grades but premium, because it cost more. There’s something to be said about the principle that so-called “price gouging” is not really a bad thing in a free market.

    For anyone who drives a vehicle that requires high-octane fuel, I recommend keeping a couple of bottles of octane booster in your vehicle (trunk, whatever) in case you end up somewhere along the route that only has regular (lots of places in the South are like this) and you’re running on fumes. Better than risking the pinging and damage that too low octane can cause for some engines.

    As for “a half tank of bad gas every month,” I think you underestimate the mixing that occurs when you fill up the tank. Every time you fill up, you dilute the old gas with the new. The bad gas doesn’t just sit around or settle to the bottom or something. It’s true that you will always have a small percentage of older gas floating around in there, but it’s not very big and not likely to be a problem as long as you fill up fairly regularly. It takes several months for gas to “go bad,” so as long as it’s not sitting for a year, you’re probably fine. No need for fuel stabilizer.

    To borrow from another field of science, the rough rule of thumb is that it requires about 4-5 “half-lives” to effectively stabilize a system when new material is being added on top of old that is being taken away. At least that’s how it works in the field of medicine, and I see no practical reason why it would be different in automotive science. If you fill up at a half-tank mark, and you fill up every two weeks, the “half-life” of that gasoline in that car’s fuel system is 2 weeks. If you multiply that out by 4-5x, you end up with 8-10 weeks. That’s how old the oldest bits of gasoline that exist in significant quantities in the tank would be. If you fill up monthly, you’re looking at 4-5 months.

    Never mind the fact that fresh fuel has anti-oxidizing agents in it that are the gas equivalent of preservatives. When you pump new gas in, those agents will help keep the older gas from going bad.

    If you’re concerned, though, I would definitely point out that this is yet another reason why you should only use ethanol-free gasoline in your tank. Besides the corrosive effects ethanol has on fuel systems (even new ones) and rubber hoses, it also makes the gas go bad sooner. It is less stable. Plus you’ll get better gas mileage and it’s better for the environment than ethanol. Ethanol is worse about absorbing water from the air than real gas is, too.

    For any and all who are interested, I did a little searching to find out what it is that makes good gas go bad (and no, it’s not the corrupting influence of power).

    Gas is a mixture of various hydrocarbons. Many of them evaporate at room temperature, and these more volatile components are also the most combustible parts of gasoline. Thus old gas is relatively depleted of these components and doesn’t burn quite as well. Second, oxidation of hydrocarbons (combining of gas and oxygen) degrades the quality of combustible materials in the gas and makes them leave more deposits in your engine and fuel lines. Third, gasoline absorbs water from the air, and water can cause rust to develop on metal exposed to this gasoline. All of these factors are mitigated by keeping gas in an airtight container with minimal oxygen in it. By the way, a similar process happens with diesel fuel.

  7. Ah, thanks, mithrandir. I didn’t notice it after mousing around with it for awhile. The resolution and brightness on my screen isn’t the greatest so it was almost invisible, plus I was tired.

    Eric_G wrote, “some manufacturers use fuel as a heatsink for the fuel pump”

    That makes sense. Not that it’s a good thing.

    So if the fuel pump is located at the top of the gas tank, no worries about that then… Running on empty is a-ok?

    • I try to keep a full tank, or as close to it as possible, for the following reasons:

      Less internal condensation and associated fuel system problems (such as internally rusted lines).

      Probably helps avoid all the loose crude from being sucked into the pump – and into your fuel system.

      More weight on the rear end – important in a 2WD pickup or a FWD car, especially. Better traction/handling – even if it does mean the vehicle is a bit heavier and so maybe a bit less quick and a bit less fuel-efficient.

  8. I’ve always been told to keep the tank above a 1/4 tank to keep the fuel pump submerged. As the story goes, some manufacturers use fuel as a heatsink for the fuel pump and it will fail sooner if you regularly run on empty.

  9. You can adjust the size of the response window. Click & hold the lower right corner. Then move the corner to the size that you need.


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