Filling tank at half

A friend of mine is panicky about running out of gas so fills when the tank is at half . This usually takes her about a month to burn a half tank. Now she fills up and already has a half tank of month old gas and then a month later fills and has 1/4 tank 2 months old , 3/4 one month old etc. until eventually you have gas that is a year old , 11 months old etc. You get the picture. With the minimum 87 octane and short shelf life , this seems to be bad practice to me. Does this make sense to you guys or am I too concerned?

The gas is constantly mixing itself old with the new so the numbers aren’t that static.
If she’s filling up once a month on a half empty tank it shouldn’t be a problem.

Fresh gas is best.

Old gas mixed with new isn’t freshened up per se… And some parts of the gas are very old. Just like old motor oil, you need to purge all the old oil as is practical. With oil, you drain it. With gas, you traditionally burn it.

She is unlikely to have any serious issues, but she is losing octane and also adding a little varnish to the engine.

As a related aside, there’s an old meme among Y2K-type survivalists and emergency personnel that you should never let your tank get less than half-full. The idea is that, if the shit hits the fan, you can get to wherever you need to get to. Whether that’s your secret militia bunker in the woods or to the fire station for work.

While you’re here (“you” being “people who have more knowledge about cars than I do, I having roughly that of a sentient carrot”), I’ve got a friend who insists that the fuel pump works harder and wears out faster when her tank is lower, so she keeps her tank 1/2+ full for that reason. Is she nuts? Her vehicle is a pickup, if that makes any difference.

In my personal experience it’s best to keep your tank as full as possible. The fuel provides cooling to the fuel pump, found in the tank on most modern vehicles. I don’t stop every three days to top off my tank, but I do not let it get below a quarter of a tank.

I suppose in more moderate climates it’s not as important. But on the whole, I see nothing wrong with this behavior.

Even though a very tiny bit of the gas would be very old, the same is true if you’re “normal” and fill up when a gallon or so is left.

I don’t have time to do the math right now, but intuitively seems to me that the average age of the gasoline in your tank would be about the same regardless of whether you filled up every month or every two months.

Assuming that gasoline goes stale fastest at the beginning and then slower later, you might be better off filling up your car more frequently.

Since the portion of the fuel in the tank that is n months old is [sup]1[/sup]/[sub]2[/sub][sup]n+1[/sup] of the total, which soon gets to be a pretty small proportion, I think your friend shouldn’t worry about it.

Is that it for comments?

I think you are close Usram. The problem is you have that small portion you mentioned [2 cups after a year ] backed by 4 cups after 11 months . Your idea only applies if you empty the tank down to a gallon or so before filling it dont you think?This is bothering me for whatever reason so dont quit.

Depends on how much you leave. If you leave a half tank to get old, and then mix it so that you have 50:50 old/new, that is much different than leaving 1 gallon and adding 20.

To restate my original point, sure you lose something with older gas, but it ain’t worth worrying about. We’re talking a month, and while gas ages and varnishes over time, I’d start getting uncomfortable if gas routinely sat there for 90 days.

When I was doing car research, the advice I read was to never let the gas get below about 1/4th a tank since any small particles of sand or whatever has gotten down into the tank will be more likely to make their way into the engine.

Personally, I’d say that’s more likely to be important than that some mathematically minuscule amount of your gas is a few months old.

They would still have to get by the filter, and the intake for the fuel line IIRC is still at the bottom of the tank, so I don’t think this matters at all. The Car Talk guys have said this concern is bunk.

Okay, this is crazy. We have plenty of real car experts on the Dope, where the heck are they?

Rick? Yoo-hoo! Somebody get **Rick **in here!

I ruined a carburetor by nearly running out of gas. When it got very low, it dumped a bunch of crud into the carburetor and it never worked right again. It did have a gas filter, but that didn’t save it.

There is a minuscule amount of difference in the head required due to gravity if the level is lower (if more gasoline is in the tank, then if the pump has to maintain a constant pressure level, it needs slightly, ever so slightly less energy to get to that level). In terms of pump wear and tear, it’s probably not even measurable outside of a laboratory with serious, old-school scientists (the kind that you had in the 1950’s, with horn-rimmed glasses and pocket protectors, who worked 12-hour days to make better products for our American Way, and consumed copious amounts of meatloaf and Schlitz beer every night.)

In that I used to manufacture fuel tanks (but not the fuel senders themselves), I can vouch that some of these original horn-rimmed guys are still around.

In any case, your owners manual is always a good reference. Most of them say something along the lines of “don’t run out of gas; it’s not good.” The horned-rimmed guys that design the pumps account for things like gravity, temperature, and so on, and so it’s just plain silly to worry about anything more than running out of gas.

As for “crud” in the tank, well, your sender is at the bottom of the tank, where gravity makes all the crud go too. There’s no difference if your tank is near empty or not.

Don’t you think we should take into consideration the cost of going to the gas station more often than need be? For example, if you can extend the life of your filter by 6 months by spending 4 extra hours at a gas station, well, I’ll take a new filter please.

If there was crud going into your carburetor it would have had to make it past the filter first. Either your filter was faulty of something else killed your carburetor. If there was a lot of gunk, wouldn’t it have clogged the filter and stalled out the car?