is it true that a full tank gives better mileage?
 

I received this article via email. I don't know if you would agree. any feedback guys? the debate here is extra weight vs the evaporation of gas!
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Pumping gas.... good things to know.

I've been in petroleum pipeline business for about 31 years, currently working for the Kinder-Morgan Pipeline here in San Jose , CA . We deliver about 4 million gallons in a 24-hour period from the pipe line; one day it's diesel, the next day it's jet fuel and gasoline. We have 34 storage tanks here with a total capacity of 16,800,000 gallons. Here are some tricks to help you get your money's worth:

1. Fill up your car or truck in the morning when the temperature is still cool. Remember that all service stations have their storage tanks buried below ground; and the colder the ground, the denser the gasoline. When it gets warmer gasoline expands, so if you're filling up in the afternoon or in the evening, what should be a gallon is not exactly a gallon. In the petroleum business, the specific gravity and temperature of the fuel (gasoline, diesel, jet fuel, ethanol and other petroleum products) are significant. Every truckload that we load is temperature- compensated so that the indicated gallon gage is actually the amount pumped. A one-degree rise in temperature is a big deal for businesses, but service stations don't have temperature compensation at their pumps.

2. If a tanker truck is filling the station's tank at the time you want to buy gas, do not fill up; most likely dirt and sludge in the tank is being stirred up when gas is being delivered, and you might be transferring that dirt from the bottom of their tank into your car's tank.

3. Fill up when your gas tank is half-full (or half-empty), because the more gas you have in your tank the less air there is and gasoline evaporates rapidly, especially when it's warm. (Gasoline storage tanks have an internal floating 'roof' membrane to act as a barrier between the gas and the atmosphere, thereby minimizing evaporation. )

4. If you look at the trigger you'll see that it has three delivery settings: slow, medium and high. When you're filling up do not squeeze the trigger of the nozzle to the high setting. You should be pumping at the slow setting, thereby minimizing vapors created while you are pumping. Hoses at the pump are corrugated; the corrugations act as a return path for vapor recovery from gas that already has been metered. If you are pumping at the high setting, the agitated gasoline contains more vapor, which is being sucked back into the underground tank, so you're getting less gas for your money. Hope this will help ease your 'pain at the pump'

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Truth!!!

I'm not gonna read your entire post - I'm just going by the title, okay?

If you look in my fuel log, you'll see that I tried running partial fills for a while - albeit on a near empty tank - and my FE dropped like a rock!

I haven't tried partial fills on a near full tank yet - but I can tell you that running my tank low gave me mileage for sh!t, if that helps...

In other word(s) - FAIL!!!

Now, I'll read your post... ;)

Okay, I read your copy n' paste...

Here's my judgement, and you can take it from the smartest guy on this web site!

1. False
2. False
3. True
4. True

And, if you don't think I'm smart, look at the BBCode I used for this message... ;)

Ok, ego aside....

I have noticed a very different result with my Metro. I get better FE the further down I ride the tank before filling up again. I do not know why BDC and I would have such different results. It could be a weight factor for my Metro, or a different fuel delivery system.

DISCLAIMER: I do NOT use scangauge, so my results are pump based...

Um...

See bullet point #3 above, Will. :thumbup:

Maybe your gas doesn't evaporate like in AZ - one of the hottest places on Earth!

No, Tennessee is not near as hot as AZ, but I would not think that would be the total deciding factor...

Well, I suppose I could recreate the experiment with a semi-full tank, instead of near empty - for 1000 miles (fill it up every day for two weeks), and I'll report back.

I tried it on an empty tank 'cause I was trying to save weight, but my mileage took a noticable down-turn!

Maybe you're getting better mileage on shorter tanks or longer tanks because of how your average mileage is working. If you do all highway driving on the first half of tank, your FE will obviously be higher than if you do hwy the first half, then city the second. Same thing with Will. Averages?

I only use my Metro in commute to and from work, so my % never changes. I am at 70% state road and 30% city.

It seems counter-intuitive that you would get better gas mileage on a full tank. As the fuel in the tank is being burned, the car is decreasing in weight (unless the occupant is eating a bunch of Big Macs.)

So it stands to reason that the car would get better economy while the tank was getting lighter and lighter. A gallon of gas weighs approximately 6 pounds (2.72kg), my tank holds about 13 gallons, and I usually fill up 10+ gallons at a time. If my math serves me correctly, this translates to a 60+ pound (27.2kg) swing in weight. Last time I checked, reducing the weight in the car increased economy.

I don't have any experience with repairing gas tanks or fuel pumps, but unless the pressure on the fuel pump at a full tank is significantly higher or lower than at half or lower, or the percent weight distribution has a major affect mileage, then there would be absolutely no reason to believe that you get better economy on a full than a partial.

It would seem that you would do better on a near empty tank, due to weight. However your car might also ride a tad lower with a full tank, reducing wind profile. How much depends on the car.

Part of the problem here is lack of any standard test or elimination of variables (air temp, pressure, humidity, wind).

Might I suggest an experiment? Get some accurate fuel consumption instrumentation that can tell you accurately the distance and fuel used, do an A-B-A test with just the weight.

1. When your tank is low, go to your test loop. A calm day would be best. Car should be already warmed up.
2. Bring along enough weight to simulate a full tank, in the trunk.
3. perform the loop with a consistent driving style:
   1. with the weight
   2. without the weight
   3. with the weight (like A)

The distance should be consistent between all runs and the fuel used *should* be different without the weight, if weight of the fuel is a factor.

re: vapors, some smart person should probably figure out the equivalent liquid gallons of 10 gallons of fuel vapor too.

yes, I can hit the list buttons too :rolleyes:

This was also discussed on snopes.com. Search the site for Kinder-Morgan.

It looks like all they did was argue about their beliefs though :)

I say test it or shut up ;)

Here are some more "facts" to help figure it out:
New Hampshire's Gasoline Vapor Recovery Program - Protecting the Air We Breathe

"eleven pounds of VOCs may be released for every 1,000 gallons of gasoline during fuel transfers. "

Assuming 6.5 lbs/gallon for gasoline, the volume of the vapor represents about 11/6500 of an equivalent volume of liquid gasoline. Or another way to look at it is the vapor density is 0.17% of the liquid density.

It shouldn't matter vapor wise if you put in 5 gallons in two fillups, or 10 gallons in one fillup, you are putting 0.17% VOC back into the pump (or into the air) in the form of vapor. The obvious difference between the two is the amount of weight you carry around.

According to my Scangauge... Empty is better than full. 750 daily commute segments over 1.5 years, and it's pretty clear.

56 mpg average: Full tank (Above F on the gauge)
60 mpg average: 1/4 or less
6.666% better to run near empty.


As for evaporation, don't we have sealed tanks and emissions gizmos built in to avoid exactly that?

So, if it's better to drive with a tank that's not full, it would make sense to not fill it up all the way. So, unless you have a SG that is calibrated correctly, it'd be tough to keep track of FE without filling up all the way, right?

yep. Or at the very least, run it down near empty instead of filling from halfway every time.
Also true. After 15,000 miles, mine's pretty much dialed in. Within 1% when I fill at the same station/pump, and within 3% at any other place.

I'm going to brainstorm here so bear with me!
When the gas evaporates in the tank, it is prevented from escaping by the seal on the filler cap right?? But isn't there a line running from the tank to a charcoal filter that collects excess vapours? Or have i gone wrong somewhere? Anyway, from what i can remember, the filter then lets the collected vapours into the inlet manifold when the engine is running. But if the charcoal filter was not working correctly, would there be vapours going into the atmosphere? (Was there a discussion on charcoal filteres here before??) I'm thinking that if the gasoline level is low, there is a lot more space to hold vapours and therefore increase the possibility of more escaping. But if it was going to escape, it was going to do it anyway, regardless of fuel level. Man, this is complicated!
Thing is, by keeping the tank topped off all the time, you are then visiting the gas station more, thereby doing more unneccessary trips AND filling up more, therefore returning collected vapours more often to the underground tank in the station. Which is apparently 1%. But that's 1% of any amount, small or big. Hmmm...
I think there is a certain amount of trade offs going on here and can't help feeling like we are being had with these emails that do the rounds. I mean, we don't see such emails telling us to air up your tires to max sidewall or to slow down to 50mph do we? I think there's certain vested interests with all this. No offence to the OP mind. Discussion is always good!

ollie

I do not have as much data to date studing my daily commute relative to tank level - but i would find it hard to believe that the vapors are escaping to anywhere in a sealed tank - Is there some vapor loss when you insert the nozzle to refill? is that quantity measureable?

I don't know.

As for your item #2 - Maybe it is just me - but if i see the tanker truck at the station unloading more fuel - I skip the fueling and get it later. There may be no truth to it - but that is what i do.

Item #4 - I have found that filling it on SLOW seems to work better for my car - maybe it is how the pipe is bent - but on HIGH flow it will click itsself off Way Early.

#1 has been discussed before and I don't think it impacts my tanks too much - Although the majority of my fills are about the same time of day ( on way home from work) so if there was a differance - my tank to tank variation would be small in my data.

Steve

My take on the 4 points:
1. the volume change due to temperature is minimal. Do what works for you.
2. absolutely. I avoid filling when the tanker is there.
3. see my posts above
4. pumping slowly gives a more consistent fill. Fast fill makes it click off at inconsistent levels. I'm doubtful about the other stuff about getting less for your money.

I attended a mini-class on racefuels a few years ago, put on by Union 76.

I'm not drawing any conclusions, just adding some pieces to the puzzle...

Evaporation is a concern on two fronts, but reduced volume from evaporation aside for a moment: The engineer teaching the class explained the lighter chemicals withing the gasoline mixture have the lower vapor pressure, separate out and evaporate first. Evaporation as a result of heat, vibration, or both. Those lighter chemicals, it was explained, are the components responsible for slightly reducing burn rate (increase octane numbers) and keep detonation from occuring. He also explained that underground fuel storage tanks don't vary much in inside tempature, much like a wine cave the earth helps average out the tempurture.

Since those lighter chemicals evaporate first, a race engine can start to detonate and loose power as the result of running on old fuel that has gotten warm and vibrated within the fuel tank, or fuel that has been left in an unsealed can or jug inside a warm race trailer for 3 weeks. He advised for best performance, only race with fuel from sealed cans or fuel that has been stored in the smallest container possible, sealed, in a cool dry place.

In a passenger car, the smarter ECU's sense detonation and increased coolant temp, then retard ignition timing and/or choose a slightly richer fuel map to keep the engine from hurting itself. With reduced power from less timing lead, the driver must give it 'more accelerator pedal' to maintain a given speed. Combined with the richer fuel map, FE goes down as the lighter chemicals evaporate. With a 1/4 tank of fuel, you are running your off of 'the leftovers' in your tank. That's facts mixed with some theory.

Here's what still doesn't make sense to me... on a passenger car, why is evaporation even a consideration? <thinking out loud here> We all have these evaporative closed loop systems... when I stop to fill up and remove the cas cap, the huge rush of air escapes. Unless you open the gascap, those vapors should just recondense and drop back into the fuel and remix, shouldn't they? <thinking more> Maybe the huge rush of escaping air is the lighter chemicals suspended above the liquid fuel, out of liquid suspension, where it does my engine no good, resulting in lower FE?

the last few gas stations I have been to had all of their labels they are required to have on the pump, including the one that said that the pump was temperature compensated, also the ground temp does not vary that much once you get below 5 feet like the tanks are at, as they are below the frost line.
Also, unless you leave your gas cap off the vapors are burnt and gas vapor burns, liquid fuel does not burn until it's vaporized.
one last nit pick about gas pumps and the sludge? have you not seen the filter on the pump? they used to be more visible on older pumps, but i don't see why they would install them on new pumps any more.

1. Bonk - ground temps are very consistent. Yes, it matters moreso to the people selling as their tanks aren't always insulated by the earth :D
2. Bonk - Fuel tanks pump from the bottom anyway - the place you're fueling from will have a fuel filter (I mean, there's a pump in there - they don't want crap in there).
3. Bonk - fuel is going to evaporate... but 1) Not as fast as you're thinking, 2) If you pump fuel into a 5L space of air, you're displacing 5L. If you displace 2.5L twice as often, you're still displacing the same amount of potential fuel vapors. AND weather or not it goes into a vapor recovery system (fuel pump nozzles with the corrugated plastic thinger) - that same volume of potential fuel vapor is being displaced.

The funny thing about the third myth is that it says you should especially do so when it's warm outside... But that contradicts the first point about only filling up when it's cold :p

4. Catch22 - mostly bonk. Fill up fast and there's less time for fuel to vaporize. Remember, that the amount that actually vaporizes isn't much to begin with. Personally, I fill up a little slower for pump consistency, it's not that it's metering differently, but the way it determines cutoff is more reliable (as someone else already mentioned)

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Extra weight.... I'm going to say close to, if not already, insignificant.

13 gallon tank (15 gallons, but you fuel up when the low fuel light comes on when there's 2 gallons left)... Lets say 7lb/gal.... So that's 84lbs of fuel... 84 pounds of a 3500lb car + 500 pounds of people (you aren't traveling by yourself are you?) is 1.7% of the gross vehicle weight. So if you only keep a half tank max, that's about 1% of total vehicle weight....

For fuel, Meh, I say. Fuel isn't something you're unnecessarily hauling, like the two bowling balls and assortment of diving weight belts in your trunk.... Spend that extra time to track fuel trends and figure out when it's better to fill up and what fuel economy you need to reach that goal :D

I mean, we could empty our coolant systems a little to save on weight... Run without washer fluid during bug season, add only the minimum amount of oil and gear lube necessary... Drive without shoes (there's some guy in Japan that does that with his Prius)... Light weight lug nuts anyone?

1000 words:
http://mcguckin.us/files/car/fill_level_2008_9.gif
750 daily commute segments included in this.
11.9 gallon tank = 83 (ish) lb of fuel, assuming the above 7lb / gal is correct.
3.6% of my car's total weight (2300 lb).
6% improvement in mileage.

Driving environment:
50/50 rural highway (55 mph) and medium town (45 mph and stoplights). 11 miles commuting distance. Texas, so temperatures ranging from just below freezing to 105F in the summer. Rolling hills with ~100 ft elevation variation.

Driving Style:
P&G all the way, with eoc. That's likely part of why the weight makes such a big difference in my case. I'm always accelerating or coasting. It'd likely make less difference with a steady-state driving style.

I simply note the fill level, rounded to the nearest 1/4, and the trip mpg by Scangauge. Average by fill level, then chart it.

Note:
My civic (and others of the same era, from similar complaints I've heard) does NOT like to be run completely empty. Mileage is great right up until the warning light comes on. Then it starts sputtering and stumbling in certain situations. Because of this, I prefer to run it down to NEAR empty, but not quite all the way. Sometimes I do anyway, for data collection purposes, like this.

That's awesome PaleMel!! :) That pretty much settles it (except the excruciating nuances of extracting the test methods).

So keep your tank at %25 and just add enough to get you to work and back each morning :D



Note, you can still "fill it up", just have to take it back out again when you get home, and store it safely!

I suppose you could do that. ;) I just fill it up and then make sure I run it all the way down.

For the record Mr. Melanesian, can you describe your driving style and environment?

added to the post above.

And another question: how on earth do you manage to fill your tank to 125%?

thanks for your replies.

I guess vapors would come out on your next fill. You'll notice that a tranparent smog comes out of your tank once you opened it for refueling. Also, if you make your tank empty, the more times you will go to the gas station. The more miles you'll spend. In addition, contaminants are settled at the bottom of the fuel tank. If you make your tank near empty, your engine will be susceptible to these contaminants.

The best way is to test it as what have been advised. Any more tests guys?

There must be something more going on. If removing 61 pounds (75% of 83ish pounds) increased my mileage by 6%, I'd be ripping out the extra seats, interior and dash right now... and then going on a diet.

You don't have to be a genius to figure out that more weight means that you will burn more gas, what does a full tank weigh???? DUH!

About 80 pounds. DUH!

Depends on the vehicle...

In my ride - the difference between 'F' and 'E' is 48 lbs.

Maybe it helps my FE by coasting further... ;)

LoL!

Takes me about 10 minutes to fill my tank - hurts my back from bending over so long, but I think it makes a difference! :)

Alot will depend on the car, a full tank on mine is 5% of total weight, definately noticable, you can feel the difference on steep climbs and drag racing :)

Imo small changes of weight wont have an much of effect on the average driver. PaleMelanesian is really pushing his civic with few modifications, I'm seriously impressed.

In my case, the best FE I have ever had was when there was 150+ extra lbs in the car.

Heh!

Must be an HX thing! :cool:

Yea, I'm not sure either... :confused:

Lol, does the phrase "This one goes to 11" mean anything to you guys? :)