Mileage trends with amount remaining in tank

[engineerdude]

Mileage decreases with bigger fuel-up quantity?

Help me figure this out. if I fill up at about half a tank or 1/4 tank remaining, it seems I get closer to 32-33 mpg. If I wait until the tank is 1/8 or less before filling up, I appear to get 29-30mpg. All this with a 2005 Chevy Malibu classic. I can't figure it out. My driving habits are not different. Any thoughts? I even top it off the same way every time.

[Pontiac]

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.

[Pontiac]

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.

[carlsr]

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]

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

[S8ER01Z]

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.

[cee]

..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]

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

[cee]

..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]

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