When ya fill up, you're adding a few extra lbs to the gas tank that the car has to haul around. If you keep the tank at half, thats half as much weight in fuel that the car has to haul, which would make sense that your mileage would go up. I have no idea how much fuel weighs per liter, but I wouldn't be surprised if its around 100lbs.

There are a couple of problems with keeping the tank low. First, if you're running the tank nearly dry, you've got sediment at the bottom of the tank which could cause damage anywhere between requiring to change the fuel filter, all the way up to killing your pump and fouling up your injectors. The other issue is depending on how often and far you drive, you'll spend a bunch more time getting more acquainted with who works at your local gas bar. In the long run, yes, you will save on fuel.

A couple of references:
Weight of Gasoline

Google Calc showing Kg/L

Google Calc showing Lbs/L

Using my tank as an example, quickly looking at my stats, it seems I fill up on average with 40L of fuel. So using the second Google Calc, 1.6* 40 = 64lbs. Ok, no not quite 100lbs, but still carrying that around. ;)

Nice info Pontiac, but what engineerdude is asking (and I have wondered why myself) is how come it seems like one gets better mileage from the top half of the tank (filling up when it is only half empty) rather then the bottom half (filling up when empty). I have noticed this on many occasions. One would think the extra weight from a full tank would adversely affect the mileage...but it doesn't seem to work that way...what other factors may be affecting this?

VWJunky
Nov 13, 2008
8:17 AM

it could be a gauge problem, the 2nd half of my tank seems to go faster than the top half. But I do fillup till i see the actual fuel to the rim. My gauge does not even move till after 150 miles. I run my tanks till I get the red ligth saying I am on reserve.
All that really matters is knowing when not to run out.
later
E

I believe this is due to the design of the tank. There is actually more fuel in the top 'half' volume wise than there is in the lower part of the tank. It's not going faster, there is simply not as much of it there.

..i've noticed this effect too..basically, everybody's saying "fill up before you hit the 1/4 tank mark to improve fuel economy"..but at the same time, this is a 'phantom' MPG improvement..looks good on paper (or in Fuelly..heh!), but in actuality there's no actual increase in MPG..it's all based on fuel tank design (and/or poor fuel gauge design)..

..then again, this is saying the #s lie..when you refuel earlier it's a false reading..but i can't grok how that works, since the refueling obviously means less gallons needed & the calculations show better economy..

..still confused (personally)

VWJunky
Nov 28, 2008
11:57 AM

Just go by what you actually put in your tank from Full. SO if you fill all the way up and drive 500 miles and then you fill up all the way again say 15 gallions then you can figure out your mileage. Don't go by the guage at all except for use it to figure out when to fill up again and to not run out of fuel.
I fill up till I see it and run it out till the low fuel light comes on. I do this to get a more accurate measurement and to see how far I can go on a tank.
later
E

..sensible, E, but is it truly more "accurate" to take it to Empty?..if you opt to refill at 1/4 tank regularly, and you see a mpg boost, why not refill early whenever possible?..the gal.s used & miles traveled still calc to better performance, no?..

..maybe something screwy like gasoline being more likely to vaporize to fill the emptier gas tank is going on (or somesuch oddball theory)..whatever the case, if i'm netting better mpg, i think i'd refill earlier rather than wait..

just more thoughts to ponder

VWJunky
Dec 1, 2008
5:16 AM

accuracy is in the fill up. Knowing just how much you put in, you don't have to drive the tank out (just a personal thing) you can just drive 100 miles or 1 mile and then fill up to the same level you filled it up last time. This give you really accurate measurements. This really won't get better milage but just accuracy in your fillups. (of couse if you are dealing with weight of fuel that is another story)
If you don't like fillling to the rim try using the same pump at the same gas station. over the long haul with many fuel ups it will average out with time if you are not as accurate as you want to be.
SOme crazys (me) just take it too to far when doing somethings! LOL!
Erick

In my 06 Prius I can squeeze in just over 10 gallons once the light comes on or about 60 lbs. With all the electronics in the car (and I've added ScanGauge for even more) I reset the MPG at the beginning of every tank and have noted repeatedly that for the last 1/4 tank or so I am able to get the average to go up. I have attributed this to the 50 or so fewer lbs the car is hauling around at that point.

Yes the Prius exibits the same goofy behavior on gauge movement. I know that at one point I went over 300 miles before the gauge moved off FULL. Yah I love good mileage but that's just silly.

In my wife's 2002 the thing goes from 3 bars (of 10) to one bar in about 20 miles. 450 miles on the first 7 bars, 20 on the last 3!

Due to this silliness, I pay no attention to the fuel gauge except if I plan on going a fair distance to judge if I can make without having to stop.

It doesn't need to be this way either. We had a boat for years that held 12 gallons. When full the needle exactly covered the full mark. When it was straight up at half tank exactly 6 gallons would fill it. When the needle perfectly covered the "E" the motor quit, it was quite remarkably accurate. (Yes we always carried spare fuel on the water!)

I'm guessing on a boat, packaging is less of an issue than on a car. I think most fuel gauges still use a float, although I'm not sure about that, so they're actually measuring fuel height in the tank, not fuel. If you've ever seen a gas tank (here's a pretty extreme example: http://www.ineospolyolefins.com/Images/SM%20-%20Car%20Fuel%20Tank.jpg), you can imagine that fuel level really does not track fuel volume very closely.

I'm no expert, so you can take my opinion for what its worth, but here's my take on this. Your cars fuel system is designed to run cycles from full to empty, meaning that its okay to wait until the light comes on to tell you that you need fuel, if you filled it to full to start. Manufacturers tend to agree because the manuals for all the vehicles I have ever owned simply say to not drive vehicle below E as to not run out of gas. My point is that it is okay to fill your tank up, then drive till the light comes on. Just fill up as soon as possible after that.

Partial fill-ups have two downsides, and one upside. The upside is that you carry less weight, effectively increasing MPG's. This I feel is negligible, since I drive a Titan with a 28 gal. tank, and have not noticed a difference in MPG at any level of fill up. The first downside has already been discussed. Constantly driving your vehicle with less than a 1/4 tank of gas will cause the sediment in your tank to clog your pump, lines, filter, injectors, or any combination of these components.

The second downside is what I think may lead to MPG variance based on when, and how much you fill your tank. A common misconception about fuel systems is that its a one-way feed. Gas is pumped from your tank, through the filter, to the injectors, and into your cylinders for ignition. However, fuel systems are not one-way, they are loops. Fuel is pumped from your tank, to the filter, to the injectors, where only a small portion of what is in the lines is fed into the cylinders. The rest returns back on to the tank where it awaits to be pumped back to through the loop. Fuel pumps are electric motors, and like any motor, electric or not, they generate heat. Fuel pumps are designed to cool, or at least not overheat, by using the fuel that it is pumping to absorb some of the heat. So your fuel heats up as it travels into the engine compartment and close to the engine, returns to the tank, and then heats a little bit more as it passes back through the fuel pump. This creates a never-ending cycle of a little heat, a lot of heat, then sitting in the tank to cool. If you constantly have low fuel, it won't have time in the tank to cool, therefore it will continue to increase in temperature, and at some point, on longer drives, actually cause the fuel pump to increase in temperature!

Now, if you run your vehicle until the light comes on, as long as roughly 75% of your driving time is done with more than a 1/4 of a tank, your fuel pump was designed to get hot a some point. But if 75% of the time you have less than a 1/4 of a tank, your fuel pump with be hot all the time. My friend has a 98 prelude, and refuses to ever put more than $10 at a time, usually $5. He is on his 3rd fuel pump in 3 years and he still doesn't believe me.

Anyway, sorry for standing on my fuel pump soapbox, but I know I was very surprised when I learned how all that works. As far as when and how much fuel to put in your tank. If you always go from 1/2 a tank to full, your fuel has more time to cools as it cycles through the system. Cooler fuel is more dense, and therefore most newer cars have the technology to sense the density of fuel, not having to deliver as much volume to meet the engines needs. If you fill from empty to 1/2 a tank, your fuel will not have as much time to cool, your fuel will be less dense, meaning the engine must deliver more volume in order to run. I'm no physicist, so its just my opinion, but it makes sense to me.

PS: I've hear a myth that you get more fuel for your money if you fill up in the morning or at night because the its cooler outside, making the fuel more dense. Gas pumps measure volume, so the denser the fuel is, the more gas you really get per cubic inch. Also just a myth, I've never paid attention to this, but I might do some experiments with that next summer. :)

..g2gsr93: i think the morning/night fuel-up theory was proven inconsequential (check snopes.com, i think they have something on that there)..

..the fuel-loop was interesting, as it implies (just to further my original theory) that a fuller tank can absorb heat more readily than a less-full one..and since gasoline tends to evaporate (at any temp) or go less dense (at hotter conditions), it makes sense that running towards Empty means you've got less fuel to cool the gas returned to your tank..

..granted this is a not-an-expert opinion squared (since i'm less expert than anybody & using it as a consideration point), but it's something to think on..

fun to ponder, at the least