is it true that a full tank gives better mileage?

[wagonman76]

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.

[dcb]

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

[bluesfan]

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

[dcb]

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

I say test it or shut up

[dcb]

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.

[PaleMelanesian]

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?

[cephraim]

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?

[PaleMelanesian]

Originally Posted by cephraim

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.

yep. Or at the very least, run it down near empty instead of filling from halfway every time.

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?

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.

[vtec-e]

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

[Doofus McFancypants]

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