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Discussion Starter #1
Hee hee, that's a loaded question...allow me to explain my query:

I have noticed that just after I first fill up my tank, my mileage suffers a bit. After I reach the halfway point, my mileage gets better.

Does anybody know just how heavy 87 octane gasoline is? I know gas is lighter than water, which weighs something slightly above 8 pounds per US gallon, right? But I don't know exactly how heavy it is.

Using the approximate weight of water to "guess", let's see...... 8 X 6 gallons = 48 pounds...for a half-tank....it's a guess.

But would it be plausible that a half-tank's worth of weight be a significant factor in terms of fuel economy? Outside of myself at about 220 pounds, I already carry very little in my car during my day-to-day activities...maybe only 15 to 20 pounds of work-related gear, books, files, etcetera. Occasionally, I transport friends...maybe 200 to 400 extra pounds, but typically, those types of trips are short and infrequent (to/from restaurants, etcetera), so I wouldn't expect them to significantly affect my mileage.

I'm wondering if maybe it's possible that as the back of the car (or wherever the gas tank is) gets lighter, maybe my mileage is improving? It sure seems that way to me. I'm down to 2 bars, and my tank mileage is 52.8 MPG (US gallons)! I love this, but it didn't start happening until after the halfway-bar disappeared from my gas guage display.

The way I figure it, it's one of three things:

1) The weight of the gas. A full tank weighs more, hence; requires more energy to drag around?

or..

2) The weight of the gas in the ...er back? of the car. As it gets lighter, the car noses down a bit, making it more aerodynamic?

or..

3) It's completely in my imagination?

So if it's either 1 or 2 above, should I consider maybe only filling my tank to the half-full point each time, so as to get even better mileage than my car is already able to do? I guess I'd have to fill it to the top once in awhile so as to be able to mathematically calculate my mileage. Maybe on a monthly basis?

If it's 3 above, I'm sure I can leave it to you smart folks to set me straight, and I can go back to seeing and hearing things that "go bump" in the night.

Anyhow, this is only supposition...you know, food for thought. I'm not planning to alter my fuelling strategies just yet.
 

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When flying, gasoline weighs 6 lbs. per gallon vs. 9 lbs. of water per gallon. I know the fuel used is regular leaded gasoline, but I doubt that the weight is significant.

Your reasons for worse mileage does hold some validity, but you must look at your driving patterns when comparing the two. Also, I found that the screen's display of MPG can be off by as much a 5 MPG. Don't trust the display.
 

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Discussion Starter #4
Hi, guys:

I usually fill up shortly before I park it for the day, so as to avoid a rush-rush morning. I hate those!

As far as the display, it's actually pretty close to my mathematically-calculated results. I just filled up today, and the screen showed 53.1 MPG, after having just dropped down from 53.2...grrrr! Anyhow, mathematically, I calculated 53.47 MPG. I take part in the GreenHybrid.com "real mileage" database, here:

http://www.greenhybrid.com/compare/mileage/

Anyhow, I seem to be doing pretty good with only 1600 miles or so on my car. After all, the EPA estimate for overall average is 55 MPG.

But I was curious about the possibility that maybe a full tank could affect gas mileage. I also realize that it could be that my car is gradually getting broken in as I drive through each tank, so maybe what I'm thinking is a "weight" issue is really just the car's behavior as I break it in. After all, I can drive over 500 miles on a tank of gas! (I love saying that! :twisted: )
 

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Cool site; thanks for posting the link. Just added my 2004; will have to add the 2003 when it's not quite so late at night. 8)
 

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BIF said:
I usually fill up shortly before I park it for the day, so as to avoid a rush-rush morning. I hate those!
Ok, so the first real driving you do on a typical full tank is that cold first five minutes that is notorious for getting lousy mileage. Thus your average MPG reading starts low and then sort of asymptotically (faster early, slower later) approaches the higher true average MPG for a full tank.

For example, five minutes into your morning commute after filling up the night before, you might have settled at 35 MPG. After another 10 minutes, it might be up to 45 MPG, then slowly climb from there for the rest of your tank toward your normal 53 MPG average. Sound about right?
 

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Discussion Starter #7
Hi, Robert. Thanks for the try. No, I don't think so.

I drive that first 5 minutes every morning, not just the first morning after filling the tank. My morning drive right after a fillup is usually around 50 MPG (of course, I'm only on my fourth tank now, so it's probably not right for me to say "usually" so soon).

I really did notice my overall mileage hovering around 48 or 49 (with fluctuations) up until one mark below half-full (at about 250 miles or so). At that point, it seemed to easily go up to 53, 54, or thereabouts, and stay there.

I will just try to pay better attention for the next few tanks, and I'll make any relevant notes here in this thread.
 

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Hi BIF,
Try resetting your cumulative average some evening just before shutting off the car, when the tank is half full. Then you can see if the second half of the tank really does behave differently.

Unless you do a LOT of speeding up and slowing down, I have a hard time believing the extra weight of the first half of the gas in the tank would have even 1 MPG difference. It's only around 1/50 the weight of the car.
 

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Airplane Pilots only fill the tanks as needed plus safety...

RSnyder said:
Hi BIF,
Try resetting your cumulative average some evening just before shutting off the car, when the tank is half full. Then you can see if the second half of the tank really does behave differently.

Unless you do a LOT of speeding up and slowing down, I have a hard time believing the extra weight of the first half of the gas in the tank would have even 1 MPG difference. It's only around 1/50 the weight of the car.
Good question!

I have pondered with the Idea to compare it to the travel preparation of a Pilot flying the privat Learjet. We may be talking about a lot more mass but the relation ship fuel to mass to move is probably the same.

So what I will attempt after I burn up my 5th tank of gas is gradually increasing the amount of fuel on my regular commute to work and home, starting out with one gallon, to see what effect it has. I have also noticed that by the 2/3 mark of the tank the fuel average seems to improve.

Ralph
 

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Discussion Starter #10
Robert:

Great idea. I think I will try that, but I will probably wait a tank or two.

Ralph:

Good points. Please keep us posted with your benchmarks! I'm very curious as to your results. It would be nice if somebody who's car is already well broken-in could do a similar benchmark too.
 
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