half full or half empty tank?

[chris Adamson]

Should you expect better milegae filling up when your tank gets to the half full mark? (always having atleast a half a tank). If so why? What about the extra weight of a full tank?

Also, is there any truth to filling up in the morning when the gas is cooler for more density?

[kamesama980]

as opposed to what? waiting till it's on E?

with a 20 gallon tank and 10 gallon halfway mark you'll be running about what? 7lb/gal so 70 lbs difference from full or empty....maybe 1/10 mpg gain. nothing outside the normal variation.

as for filling up in the AM: that's logical but not thought out urban myth. gas is stored many feet underground and the temperature doesn't fluctuate much down there. ever been in a cave? the mouth is close to ambient temp but get a hundred feet in and it'll vary 5 degrees between a 90f day and a 0f winter night. even in the northern US permafrost only reaches a few feet down through the soil.

[Project84]

I think the myth about filling at halfway vs. filling at E is all due to variations in gas tank designs.

My Saturn for instance APPEARS to go through gas after the halfway point VERY quickly. Take into consideration the design of the gas tank and you find that halfway isn't exactly 6.25 gallons, it's probably closer to 5 gallons or less.

[DaX]

In addition to what kamesama980 said, I think that modern gas pumps compensate for fluctuating temperatures.

99% of the time I only fill when below E, and like Project84 said, the fuel level APPEARS to drop rapidly from 1/2 tank to about 1/8 tank, then slows way down again.

[PaleMelanesian]

350 data points, on my daily commute, since April. Varying weather conditions.

125% is from topping off, when the needle is above the F on the gauge.

0% has the fewest data points, and as such is somewhat suspect. Sometimes I have to fill up from 1/2 or 1/4, for a trip or something.

Tank Fill / Avg mpg
125% / 54.0 mpg
100% / 54.5 mpg
75% / 55.2 mpg
50% / 55.5 mpg
25% / 55.5 mpg
0% / 55.2 mpg

[Thermactor]

I only fill up my tank halfway because it's an easy 100 lbs saved. And I'm poor and dont wanna fill up the whole tank.

[jadziasman]

In winter, in the freezing north - home of the climatically challenged, it's smart to keep the tank at least half full. You never know when or where an old car will decide to break down and it's really nice to have a lot of gas to burn if you need to wait for help to arrive.

[Pete]

More related to reliability then fuel economy I have always had at least half a tank simply to stop the pump sucking up the rubbish which settles in the bottom of the tank.

It also allows you the option to add fuel at your choice not that dictated by a flashing light on the gas gauge.
If you are about to run out then you pay whatever the gas station asks since you have little choice.

Also I like to fill at places having a high turnover since the fuel is normally sitting for less time than some place off the main traffic flow where the fuel may sit for ages before it is used and replaced.

By the way the differences in economy posted by PaleMelanesian amount to less then I MPG in 55 or between one and two percent.

Pete.

[chris Adamson]

The other part of the myth than spawned my question was the claim that extra airspace in the tank was a bad thing because there is too much room for vapor. Isnt the tank a closed system? So would one get better mileage on a full to half tank or half to empty tank?

[Hateful]

I get slightly better mileage with the tank half full, but after I drive out of my way to stop an extra time or have to get gas at a higher priced station, it comes out the same. I ride without the front passenger seat most of the time to make up the weight difference; once it started getting cold I didn't want to pump gas in the cold cold wind any more than I had to.
Vapors aren't going anywhere but back into the liquid when it cools off; providing it got warm enough to evaporate to begin with.

[trollbait]

Back when I had the Prius, I noticed that the average mpg for the tank would start increasing once it got down to a half. So I tried putting in 3 to 4 gals at a time, and I did see some improvement. IIRC, around 2mpg. I think there was more to it than the lessened weight. The column pressure of a full tank, compounded by an expanded fuel bladder, might have been pushing slightly more fuel in than the car was asking for. Presumably, you should be able to compensate for that by using less throttle, but ithat required degree of control may not be possible with a standard foot controlled throttle.

[theCase]

I'm in the wait-till-empty camp. I personally don't enjoy going to the gas station, especially when it's subzero. Waiting till empty cuts my visits in half.

From an efficiency position, it doesn't seem to matter, but I rationalize it that it does, as one gets better mileage driving by a gas station than stopping for gas.

[StorminMatt]

Also remember that the KIND of driving matters as well. If you are doing constant speed freeway driving on level ground, then an extra 50-100 pounds of gas is going to make little difference. In this situation, the lion's share of the power required is going to be used to overcome aerodynamic drag. But if you are doing alot of city driving or climbing hills. then the difference will be MUCH more significant. In these situations, you actually have to accelerate and/or lift that extra gas. And this costs energy. This could be specially true in a small and light car. For instance, shedding 50 pounds from a CRX makes a noticeable difference in acceleration. The same wuld probably be true with fuel economy.

[PaleMelanesian]

350 data points, on my daily commute, since April. Varying weather conditions.

125% is from topping off, when the needle is above the F on the gauge.

0% has the fewest data points, and as such is somewhat suspect. Sometimes I have to fill up from 1/2 or 1/4, for a trip or something.

Tank Fill / Avg mpg
125% / 54.0 mpg
100% / 54.5 mpg
75% / 55.2 mpg
50% / 55.5 mpg
25% / 55.5 mpg
0% / 55.2 mpg

Update: Looks like empty tank IS better. 430 data points now.
Tank Fill / Avg mpg
125% / 54.3 mpg
100% / 55.1 mpg
75% / 55.7 mpg
50% / 56.4 mpg
25% / 57.0 mpg
0% / 55.0 mpg

Again, 0% is suspect with the fewest points. Also, my car has a little stumble when the level gets too low.

[RoadWarrior]

Although the tanks are underground, earlier in the morning, you can more often guarantee that it hasn't just got filled by a tanker that's been baking in the sun all day and has probably had overnight to cool off since the last delivery.

FWIW, I don't like going under 1/4 tank, it's bad for the fuel pump.

[Pete]

The other part of the myth than spawned my question was the claim that extra airspace in the tank was a bad thing because there is too much room for vapor. Isnt the tank a closed system? So would one get better mileage on a full to half tank or half to empty tank?

Chris,

These days tanks are a closed system utilising a charcoal cannister to contain the vapours from the fuel system.
They are also made of plastic which helps moderate the effects of temperature changes when compared to metal tanks.

The concerns about water condensing from the atmosphere and diluting the fuel are a hangover from the days of unsealed systems and metal tanks.

The mileage depends on many factors and I suspect the weight of the fuel carried plays a small part , but only a small part.

Pete.

[VetteOwner]

The other part of the myth than spawned my question was the claim that extra airspace in the tank was a bad thing because there is too much room for vapor. Isnt the tank a closed system? So would one get better mileage on a full to half tank or half to empty tank?

if it were a closed system then woudlnt after you drained some gas it would stop from vacum locking? :D (every try to fill a lanmower and not open the vent cap? yea stops the flow from the can pretty easily then does that gurgle suckign in huge bubbles of air thing)

[VetteOwner]

FWIW, I don't like going under 1/4 tank, it's bad for the fuel pump.

yup very true, electric in tank fuel pumps (us carbed guys dont have to worry, mechanical pumps ran off the cam FTW:D ) use the fuel as coolant. you wouldnt fill your cooling system only 1/4 way up to save weight would you?:p

[Pete]

The system is closed not sealed.
If it were sealed then , yes , pumping some out would indeed cause a vacuum and eventually the flow would stop.
A sealed system can let in some air to replace the fuel but not let any fuel vapours escape ; at least not until the cap is removed.

Pete.

[JanGeo]

Fuel is stored above ground at the terminals and the tanker trucks get cooled or warmed as they transport it to the stations so keep that in mind also. I know in one instance on a trip to PA I got HOT gas from the pump like they were heating it up so I don't stop there for gas anymore. Feel the metal coupling on the hose as you are filling the tank and see if it feels cold or warm when you are pumping it. Since the tanks are not insulated and are not that deeply burried then there is going to be some temperature variations in the fuel but yeah just how much is really the question.

Anyone want to start measuring the fuel temperature when they fill up???

[spacepilot]

Since the tanks are not insulated and are not that deeply burried then there is going to be some temperature variations in the fuel but yeah just how much is really the question.

Let's see, the coefficient of volumetric thermal expansion of gasoline is 950 10^-6/K at 20 ?C, according to wikipedia (http://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion#Thermal_expansion _coefficients_for_some_common_materials).

If the temperature is increased by 10?C (18?F), the volume of a certain amount of gasoline increases by 1% (0.95% to be exact). As a result, you are getting 1% less gas for your money, or, equivalently, the gas price is raised by 1%.

I would agree with JanGeo that the 1%/18?F ratio probably makes it worthwhile to find out how much the gas temperature can vary.

[VetteOwner]

heh good luck tryign to get a thermometer in the nozzle while the nozzles in he tank :)

if it were a small storage container then yea i could see it heating up and cooling off daily but those underground storage tanks are huge! (like i belive 200,000 gallons or somehting crazy) it would take ALOT of heat to heat them up so you might see a difference in winter vs summer but not on a hourly basis... and from what i know, the only scource of heat is the sun and since it has to heat all the dirt and concrete even befor eit hits the tank, alof of that heat is absorbed into the ohh, top foot at the most.(try it, dig a hole in a sunny spot and feel how cold the soil is a foot down) now imagine 6+ feet down for the tank)

that and when the trucks there dumping in the gas that its stirring up all sorts of gunk and silt in the tanks wich could end up in your tank :) another reason to avoid "hot fuel"

[8307c4]

Here is my experience:

1/4 tank minimum, once she's at 1/4 it needs filling, and now.
It may get better mpg but running out of fuel is not a game I like to play.

I always fill her up, the slight difference in mpg is lost when I have to refuel more often, stops waste fuel and this includes stopping for fuel. Yes, I find I can increase my mpg by maybe 0.1 by always filling up, the car gets better mpg when I can keep on going past that fuel station (unless the tank needs it).

So I am thinking, running fuel tanks below 1/2 the minor gain from the weight saved is lost when stopping for fuel (a bit of idling and then having to accelerate back to speed after fueling).

Other notes: Underground tanks hold about 10,000 gallons.
A busy fuel station has to have fuel delivered daily, at least regular.
The sediments stirred by dumping "hot" fuel stay afloat for up to 2 hours, it might help to notice if there's a specific time when a tanker truck comes in thou this usually varies.
Tanker trucks hold about 9,000 gallons.

I only fill up my tank halfway because it's an easy 100 lbs saved. And I'm poor and dont wanna fill up the whole tank.

True, but why not prolong the interval, say if you're filling up weekly now make it bi-weekly?

[LarryClapp]

I'm in the wait-till-empty camp. I personally don't enjoy going to the gas station, especially when it's subzero. Waiting till empty cuts my visits in half.

From an efficiency position, it doesn't seem to matter, but I rationalize it that it does, as one gets better mileage driving by a gas station than stopping for gas.

It is the fuel used from 0-mph to cruise speed that is wasted by all the extra stops. Why do you figure the talk about jackrabbit starts and stop signals.

The talk about half tank is all about condensation form the warmer air in tank cooling off during night forming water. "Hot air cooling creates water (ie. RAIN) Also, good idea as Northeners will tell you in Winter for heat in break-down situations, provided that motor still runs. But that puts you to 0-mpg. The other family members won't see it that way thought.
My opinion is "What ever FLOATS your BOAT"!!
My question is "Why has Other TECHNOLOGIES ADVANCED so much more over the last 100 years and yet Automobiles HAVE NOT?"

[PaleMelanesian]

Updated again, this time with a picture. My car has a stumble when it gets low, so the 1% column is still suspect. This is from about 500 data points on my daily commute, using the Scangauge readings. (Q1 and Q3 are the 25% and 75% points, so half of the data points are in the range between those)

http://www.gassavers.org/attachment.php?attachmentid=1267&d=1209418856

[techartist]

Here's another article which might be useful to you:

http://www.creditinfocenter.com/wordpress/2008/07/24/increase-mileage-gas-tank-full-or-half-empty/

[Jay2TheRescue]

heh good luck tryign to get a thermometer in the nozzle while the nozzles in he tank :)

if it were a small storage container then yea i could see it heating up and cooling off daily but those underground storage tanks are huge! (like i belive 200,000 gallons or somehting crazy) it would take ALOT of heat to heat them up so you might see a difference in winter vs summer but not on a hourly basis... and from what i know, the only scource of heat is the sun and since it has to heat all the dirt and concrete even befor eit hits the tank, alof of that heat is absorbed into the ohh, top foot at the most.(try it, dig a hole in a sunny spot and feel how cold the soil is a foot down) now imagine 6+ feet down for the tank)

that and when the trucks there dumping in the gas that its stirring up all sorts of gunk and silt in the tanks wich could end up in your tank :) another reason to avoid "hot fuel"
Sediment in the fuel is the very reason I will not stop at a station where I see a tanker truck in the lot, or pulling out. The fuel rushing into the underground tank can also stir up any water at the bottom, and pump it into your car.

-Jay

[R.I.D.E.]

I usually go about half a tank. With potential hurricane threats here, you could end up with no gas available for over a week. I usually stock up on gas when a storm is threatening. Last one I had 65 gallons, and actually gave some to the shelter where my wife had to work so they could have lights and coffee.

My brothers house had 2 1/2 feet of water in the living room and he had to use a boat to get 2 miles to dry land. It was raised up 9 feet the next year.

Ditto on saving the fuel pump, your tank heats up as long as you drive (return loop suel systems), so the temperature of the fuel in your tank to skew your mileage readings by not allowing a complete fill up.

regards
gary

[PaleMelanesian]

^ that's a good reason to keep some gas in the tank, especially this time of year, regardless of the fuel economy effects.

[KU40]

Underground tanks don't change in temperature much if at all. The sun only warms the top 6" or so. Down where the tanks are it should stay 55-60 degrees pretty much all the time. If you touch the nozzle and it feels hot it's not because the underground tank is hot. If the gas is heated up at all it's because of the pump. But most likely it feels hot because of the ambient air temperature, especially if the pump is on the end where the sun can creep onto it at certain times of day. All the ones I remember feeling were actually cold.

Sediment in the fuel is the very reason I will not stop at a station where I see a tanker truck in the lot, or pulling out. The fuel rushing into the underground tank can also stir up any water at the bottom, and pump it into your car.

-Jay

Isn't that what the fuel filter is for?

[Jay2TheRescue]

Underground tanks don't change in temperature much if at all. The sun only warms the top 6" or so. Down where the tanks are it should stay 55-60 degrees pretty much all the time. If you touch the nozzle and it feels hot it's not because the underground tank is hot. If the gas is heated up at all it's because of the pump. But most likely it feels hot because of the ambient air temperature, especially if the pump is on the end where the sun can creep onto it at certain times of day. All the ones I remember feeling were actually cold.



Isn't that what the fuel filter is for?
Fuel filters don't filter out water...

[Lug_Nut]

Ummm.... Isn't the fuel pick-up near the bottom of the tank (cars or the underground storage) anyway?
The concern about filling the underground tank with a fresh load and 'stirring up' sediment in it doesn't make sense if the bottom of the tank is from where all the fuel for all those prior cars has come. Any sediment that settled at or near the bottom would be long gone.
Ditto for the car's fuel tank. The fuel and any heavier than fuel debris at the bottom of the tank is the first to be drawn to or into the fuel lines. The action of driving itself continues to mix the contents of the fuel tank continually keeping the debris moving and available for pick up.
Chain saws, string trimmers, other multi position motors that use a floating pick-up to prevent fuel starvation when operated inverted are obviously not part of this group.

A 12 gallon tank filled when the level drops to 2 will have 6 stops for fuel, while filling when the level drops down to 6 remaining will require 10 stops for the same 60 gallons of fuel consumption. The extra time to put in the four added gallons of fuel those six episodes is small in comparison to the time to pull in, uncap the tank, pay, start and drive off the four additional stops.

The fuel filters in my diesels do have a water separator chamber at their base that is a scheduled maintenance item. Each 15k miles the drain is to be opened for the content to be drained (and examined). I haven't ever seen any, fwiw.

[KU40]

Fuel filters don't filter out water...

oh, whoops I glazed over where it said stir up water in the UST. Was talking about sediment in the beginning and I guess I assumed you were still talking about it at the end of the paragraph. That was after a 16 hour work day, I was kinda tired. :)

[Jay2TheRescue]

The fuel pickup is near the bottom, but not at the bottom so you don't directly suck up water and sediments. Sure, fuel filters filter sediments out, but if you can avoid the sediments and water by not fueling if you see a tanker truck in the lot, why not? At the very least, you are paying for water and sediments at over $4.00/gallon and you either filter them out, or they make your car run like crap.

In the grand scheme of things say you get a 15 gallon tank of fuel while the tanker truck is there. Let's say that 1/2 of 1% of that 15 gallons is water. That's 0.075 gallons of water. Multiply this by $4.00/gallon and you've paid $0.30 for water you can't burn, and you can't drink. When I was in high school I used to work for an air conditioning company. The owner of the company was from Kentucky and had a saying... Never spend your money on anything you can't eat, burn, or screw.

[R.I.D.E.]

I would think the 10% ethanol here would emulsify any water in the tanks, just like the additives you used to use for the same purpose.

regards
gary

[Ford Man]

I seldom go below a half tank anymore. I have had to drop the fuel tank on my '88 Escort a couple of times to replace the fuel pump and the actual pump is near the halfway point. As others have pointed out the fuel in the tank is also used to keep the fuel pump cool. Each time I have had a fuel pump go out the tank was at or below 1/4. The extra stops are well worth the trouble compared to having to drop the fuel tank and replace the fuel pump, although lots of cars now have access to the fuel pump by simply removing the rear seat.

[trollbait]

I would think the 10% ethanol here would emulsify any water in the tanks, just like the additives you used to use for the same purpose.

I was thinking that also, but it was mentioned in another thread that water will come out of solution at 97% or less ethanol when the alcohol is mixed with gas. Pure ethanol is also hygroscopic. It will suck moisture out the air it is in contact with until it reaches 95.5%. So pure ethanol, if it chance to absorb water, will release some of that water when mixed with gasoline.

Perhaps it can still emulsify the water, but that isn't as good dissolving the water in terms of getting it out of the tank.