Gas: Full up or half a tank???

JohnnyDaJackal · 01-02-08, 01:59

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?

BigJimZ28 · 01-02-08, 02:13

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

etml12 · 01-02-08, 02:33

Quote:

Originally Posted by BigJimZ28

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

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.

jimmie jam · 01-02-08, 03:12

Quote:

Originally Posted by BigJimZ28

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

i agree. fill it up when it gets to 1/2+-.

BlackElantraGT · 01-02-08, 03:12

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.

shimp · 01-02-08, 03:26

use an additive in yer tank, such as seafoam, and dont worry bout the moisture

Accumulator · 01-02-08, 03:38

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.

Puckman · 01-02-08, 04:04

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.

bert31 · 01-02-08, 04:10

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.

BigJimZ28 · 01-02-08, 04:13

Quote:

Originally Posted by bert31

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.

I keep it full and add sta-bil

Accumulator · 01-02-08, 04:18

Quote:

Originally Posted by bert31

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..

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.

P1et · 01-02-08, 04:54

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!