Gas: Full up or half a tank???

J

JaCkaL829

it was my first time...
I have a question for the Autopian engineers and car guys. I'm wondering what's more beneficial, to keep your gas tank at full (i.e. once you get to half a tank, fill it up again) or to ride the bottom half of your tank (i.e. only fill it up to around 1/2 a tank)???



Ever since I got my new car, one of my peeves is the gas gauge is off, especially at the bottom half, so I usually fill it up once I'm at or around 1/2 a tank. I don't like to chance it since my tank isn't the biggest either. I remember when I first started driving one of neighbors was advising me to always do that because it's bad to run your tank empty because of dirt getting in. I really don't know about the dirt, but I feel it wouldn't be to good on the engine to run it almost dry. Well just a couple nights ago my best friend, who is also the most argumentative person I know, was proposing that it's best to never fill it up to half, because anymore and it's extra weight on the car. I understand what he's saying, but I still feel running it low can cause negative effects to the engine. From what I understand it's good to keep at least 1/4-1/2 a tank in the winter months because of the possibility of gas lines freezing up; but I'm wondering about the other months.



So my question is which is the preferred method and why?
 
keep it full



water is attracted to gas and there will always be some water in the tank



if it gets too low there are several problems

1.)that water that is in the tank that normally floats will try to pass through the engine

2.) there is usually sedimate in the tank and it can get stuck on the in-tank filter and cause low fuel pressure



3.)and the big one that most people don't think about

the gas keeps the fuel pump cool the more gas the longer the pump will last
 
BigJimZ28 said:
keep it full



water is attracted to gas and there will always be some water in the tank



if it gets too low there are several problems

1.)that water that is in the tank that normally floats will try to pass through the engine

2.) there is usually sedimate in the tank and it can get stuck on the in-tank filter and cause low fuel pressure



3.)and the big one that most people don't think about

the gas keeps the fuel pump cool the more gas the longer the pump will last
Click to expand...



For the most part I agree. Except that point #1 is reversed. Gas has a low specific gravity and thus floats on water. I think it is more likely that water getting into the fuel when the level is low is simply because there is a large volume of air. The larger airspace means more water vapor which can condense and then sink to the bottom of the tank.



Also, how exactly does water get attracted to gas? They are immiscible substances unlike say brake fluid which is hygroscopic.
 
BigJimZ28 said:
keep it full



water is attracted to gas and there will always be some water in the tank



if it gets too low there are several problems

1.)that water that is in the tank that normally floats will try to pass through the engine

2.) there is usually sedimate in the tank and it can get stuck on the in-tank filter and cause low fuel pressure



3.)and the big one that most people don't think about

the gas keeps the fuel pump cool the more gas the longer the pump will last
Click to expand...



i agree. fill it up when it gets to 1/2+-.
 
I'm no rocket scientist so for me I try to keep my tank full, and then run it to less than 1/4 or close to E before I refuel. I'm a heavy set guy so the weight savings from only having half a tank of fuel is negligible. If I wanted to save weight I could shed some pounds off myself first. It's not like I'm going to be drag racing my car and worried about shaving an extra hundredth of a second off my 1/4 mile time.



Plus, with gas so expensive, I try to drive around as least as possible to find gas so it wouldn't make sense to make twice as many trips just to get gas.
 
OK, if you're drag racing I can maybe see it...but for over-the-road driving running out of fuel can literally be fatal.



I worry less about moisture/sediment/etc. (never had a problem) or fuel pump life or any of the other stuff...I worry about the vehicle being ready to get me wherever I need to go.



I tend to treat the half-tank mark as if it were "empty". [stuff] happens, and running out of fuel isn't something I want to experience.
 
Gas weighs 6 pounds per gallon. If you're down 8-10 gallons and running on half a tank you're weight saving is only about 60 pounds. Not enough for me to consider as a benefit. I like to keep my fuel pump fully submersed and under a little pressure so I usually fill up when I reach 1/2 to 1/3 tank.
 
What about if you own a garage queen that gets less than 100 miles a month on it? I would assume you would not want gas sitting in the tank for over a month, I believe it would start to crystalize. In that case, I would be tempted to only fill it up half way then top off at the quarter mark. That way gas is being circulated enough.
 
bert31 said:
What about if you own a garage queen that gets less than 100 miles a month on it? I would assume you would not want gas sitting in the tank for over a month, I believe it would start to crystalize. In that case, I would be tempted to only fill it up half way then top off at the quarter mark. That way gas is being circulated enough.
Click to expand...



I keep it full and add sta-bil
 
bert31 said:
What about if you own a garage queen that gets less than 100 miles a month on it? I would assume you would not want gas sitting in the tank for over a month, I believe it would start to crystalize..
Click to expand...



The gas in the Jag sits there for a *lot* longer than a month...actually a month is nothing for my vehicles, many of them sit for that long at a stretch. I've *never*, not once, had a problem with gas in an auto going bad and I've never used any stabilizer additives.



No, I'm not recommending that others tempt fate as I have, but it's just never been a problem and I've been letting cars sit for months on end for decades.
 
I have a '97 Dodge that has had the same fuel sitting in it for almost two years now. I haven't started it yet, but am hoping it will be just fine.



The above post, however, scares me a bit!
 
JohnnyDaJackal said:
I have a question for the Autopian engineers and car guys. I'm wondering what's more beneficial, to keep your gas tank at full (i.e. once you get to half a tank, fill it up again) or to ride the bottom half of your tank (i.e. only fill it up to around 1/2 a tank)???



Ever since I got my new car, one of my peeves is the gas gauge is off, especially at the bottom half, so I usually fill it up once I'm at or around 1/2 a tank. I don't like to chance it since my tank isn't the biggest either. I remember when I first started driving one of neighbors was advising me to always do that because it's bad to run your tank empty because of dirt getting in. I really don't know about the dirt, but I feel it wouldn't be to good on the engine to run it almost dry. Well just a couple nights ago my best friend, who is also the most argumentative person I know, was proposing that it's best to never fill it up to half, because anymore and it's extra weight on the car. I understand what he's saying, but I still feel running it low can cause negative effects to the engine. From what I understand it's good to keep at least 1/4-1/2 a tank in the winter months because of the possibility of gas lines freezing up; but I'm wondering about the other months.



So my question is which is the preferred method and why?
Click to expand...



By the way, I've seen your pics on POTN and they are quite impressive!
 
Johnny, most cars have non-linear gas gauges. Pat Bedard did an editorial in C/D on this years ago, he talked to some auto engineers who said it's because people don't want to get depressed by the gauge going down fast after they fill up. I'll agree that RSX's are pretty bad in this regard.



The amount of gas in the tank has no bearing on the likelihood of picking up sediment from the bottom of the tank, as that will always be at the bottom of the tank anyway. If you run the tank way down and you have a lot of water floating on top of the gas, yes, you can draw some water into the fuel pickup, and if you're unlucky you might stop the car with it in a place where it could clog a fuel line if it froze (and yes you could get stalling or hesitation if you injected a slug of water into the engine).



As noted before, the water usually gets into your tank with air as the fuel level drops in the tank. Under the right conditions, the moisture in the air will condense onto the walls of the tank and run down onto the fuel, where it will stay. You can also get water from the tank at the station, if they aren't monitoring the water level on top of the gas and run down to the pump suction level, or if you fill up during/after a tank truck fill when the gas is all stirred up with the water and previously mentioned sediment (never get gas if you see a tank truck near the station).



I'm kind of curious what kind of "dri-gas" effect is now prevalent since the MTBE additive has been replaced by ethanol and whether that has a water-absorbing effect like methanol and IPA.



To address a couple other points, when I had my "mold-mobile", I would only drive it about every six months, so the gas in there could be a couple of years old, and like Accumulator, I never had any problems...but as noted, probably not a situation to emulate. I also tend to run my tank down kind of low, mostly from a time efficiency standpoint to make less trips to the gas station, but also a bit of the weight issue in the back of my mind....I've also never had a gas line freeze.
 
I have the same problem, my Civic since new has read 1/5 1/6 of a tank full when I start driving and after several minutes it reads empty and the light comes on. I've usually filled up when the light comes on and sometimes it takes a couple tries to start when it reaches that low. It is annoying to fill up half tanks so I like waiting till I need to fill it up.
 
Thanks for the replies, I guess it really doesn't matter. I will probably continue to top my tank off though because as I said before the second half of my gas gauge seems to go down quite fast, and I like to monitor my mpg everytime I'm at the pump.



P1et said:
By the way, I've seen your pics on POTN and they are quite impressive!
Click to expand...



Thanks, I really appreciate it. I'm surprised you recognized them though because I really didn't post that much, and whatever I got posted didn't get to much attention (there are lots of good photographers on that site).
 
JohnnyDaJackal said:
Thanks for the replies, I guess it really doesn't matter.
Click to expand...



Well there's no two ways about it, keeping the tank fuller will lessen the amount of condensation (water) in the tank, whether that makes an operational difference or not is another story. An occassional shot of Heat or Dri-Gas can't hurt.
 
If you go below 1/4 of a tank it will shorten the life of the fuel pump and they aren't exactly cheap. But I'm sure you could do the math to figure out how much gas you would save from not carrying that extra weight and it would probably be more than the fuel pump.
 
Setec Astronomy said:
If you run the tank way down and you have a lot of water floating on top of the gas, yes, you can draw some water into the fuel pickup, and if you're unlucky you might stop the car with it in a place where it could clog a fuel line if it froze (and yes you could get stalling or hesitation if you injected a slug of water into the engine).
Click to expand...



Gasoline floats in water, so the water will be at the bottom of the tank, not the other way around. As soon as you get water in the tank, it's the first thing that gets sucked in the engine so having either half or full tank doesn't matter. You need to avoid getting water in the tank in the first place.



Ethanol actually bonds with water, and therefore gasoline with ethanol can "absorb" a certain amount of water.



Gasoline by itself has a very low freezing point, anything from -90F to -200F. It varies because of the different compounds in the gasoline. If there is water not fully absorbed in the gasoline, this will crystallize at 32F - however, water is rarely a problem in gasoline tanks.



I don't see any problems with keeping fuel in a car tank for long periods of time, unless temps drop below -90F.
 
hockeyplaya13 said:
If you go below 1/4 of a tank it will shorten the life of the fuel pump and they aren't exactly cheap.
Click to expand...



I regularly go below 1/4 tank and I had my last car for 16 years/125K miles with no fuel pump replacement :nixweiss
 
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