half full or half empty tank?

[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.

[GasSavers_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]

Originally Posted by 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.

[GasSavers_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.

[GasSavers_Pete]

Originally Posted by 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?

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]

Originally Posted by 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?

if it were a closed system then woudlnt after you drained some gas it would stop from vacum locking? (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]

Originally Posted by RoadWarrior

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 ) use the fuel as coolant. you wouldnt fill your cooling system only 1/4 way up to save weight would you?

[GasSavers_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???