A few months ago an engineer friend mentioned to me that he'd heard that cars tend to get better mileage with a fuller gas tank. In other words, that the mileage obtained with a 1/2-full to full tank will be better than that experienced when the tank is less than 1/2 full. He believes this, and gave as a reason something relating to pressure, the full tank creates more pressure of some sort (vapor? I don't remember), which improves efficiency.

I thought this was probably a bunch of, shall we say, "hooey" (to be polite). But interestingly, now that I've been paying attention, it seems that for my last 3 or 4 tanks, I've experienced exactly that result! I have averaged between 36 and 40 (yeah, 40!) mpg for the first 100-200 miles of each of my last tanks, with the average FE going down slowly but steadily over the remainder of the tank. It almost doesn't seem to matter where I'm driving, and I do try to use the same kinds of techniques (DWL, easy acceleration, anticipating stops, no A/C, windows closed when driving over 45 miles per hour, etc.) all the time.

Anyone else experience this? Anyone know if/why my friend's theory is true?

I think after this tank (avg. FE = 40 mpg at 100 miles, 37 at 150, downhill from there, currently at 34.4 after about 200 miles), I'm going to try doing a few "small" tank fillups, running about 150 miles and topping it off, to see if I can actually average over 37 mpg for a full 4-500 miles that way.

This does not really answer your question, but if your tank is at least 3/4 full, less gas evaporates.

61.5mpg lifetime - 82mpg in recent months

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So does anyone here have a good explanation of how modern vapor recovery systems work, or know how to calculate how much gasoline can saturate 10 gallons of air?

I don't think that it really matters. The real question to me is if the gas guage is linear or not? If it is then that would show if you get better mileage on a fuller tank and would provide a useful tool in estimating how much gas is left in the tank. I personally don't see how the amount of gas in the tank can effect the gas mileage as the fuel pump controls the pressure at which gasoline is provided to the engine. The amount of fuel in the tank should not matter at all - if it did then you could mount a 200 gallon tank on your car and get really great mileage for the first few miles - but sorry that doesn't work. The engine requires a certain amount of pressure in the fuel system. To get that pressure the fuel pump has to provide the fuel under pressure to the engine. I wonder how these "Urban Legends" get started sometimes. As for my Prius, it has a bladder inside of the fuel tank and very, very little of the fuel can evaporate regardless of how much fuel is in the tank. Virtaully all cars built in recent years have pressurized fuel tanks that prohibit evaporation of the fuel. That is why you are supposed to remove the gas cap slowly and you hear that noise when you do . Evaporation used to be a minor problem but that was when gas was $.25 per gallon. The really big question is why doesn't gas still cost $.25 per gallon but that is probably in a different thread.

It is also a known fact that from the 1950s to the 1980s, maybe later too, car companies engineered fuel gauges to stay longer at the top of the gauge, with the intent being to give new car buyers a "warm fuzzy" about how great their new car was doing on gas mileage, so they could pass on word of mouth to their friends. That's true.

It is ABSOLUTELY NOT TRUE that you actually get better mileage when the tank is fuller.

My ex-wife and I had this argument once and she lost. She had actually thought that since the needle "barely moves" for the first 100 miles or so, then you can get better mileage by keeping the tank topped off !! Her Dad said so !! (long sigh)

Blondes, what can you say?

Gasoline is a light molecule, so it is volatile...meaning it has a high vapor pressure...the amount of pressure in a container due to the vaporized material relative to other materials (the atmosphere- N2, O2, CO2...). That's why it smells so strong and evaporates so quickly...and goes BOOM so easily. Diesel, having a low vapor pressure, does not normally ignite when a match is thrown on it. Care to try that with gasoline?

A container at constant total pressure (your gas tank) can hold a more or less fixed amount of stuff. If instead that volume is filled with liquid (or solid), then not much gaseous/vaporized stuff can fit. Your fuel remains in the liquid state, useable for driving.

If its mostly not filled with liquid, then there's a lot of space to fit a lot more vaporized gasoline...not useable for driving but great for adding to envorinmental problems. That's why vapor recovery is important.

Now, as far as getting better fuel efficiency, that's not what we care about. A full tank weighs more...higher pressure at the tank bottom where the pump takes suction (think about your ears at the bottom of a pool). The fuel pump adds pressure to that, and eventually its squeezed into your cylinder where it vaporizes as it is released into the low-pressure of the cylinder chamber and ignites.

So a full tank can deliver slightly higher fuel pressure to the cylinder, so when the gas gets to your cylinder, it evaporates more easily. Less energy required to make it go boom means more energy available to go roundy-roundy. There's more to it than that, but there's part of the just of it. Same is true for any pumping system- the more "head" you can deliver without having to acutally pump, the more efficient the system is.

A full tank also compresses the fuel ever so slightly, meaning more gasoline 'fits' in a gallon at the bottom of the tank when full than empty...we're talking a very small amount, though. Hydraulics guys, don't kill me here- gasoline is not a perfectly incompressible fluid. On the micro scale it does pack in there more tightly.

I'm not sure how much difference it makes, but its for real. Methinks you gotta have some very precise instrumentation to see the effect on the scale of a single car engine.

DISCLAIMER: This is the THEORY. I don't go around measuring fuel manifold pressures on full/empty tanks, so I cannot back it up for cars. Ships, yes. Cars, no. As I said, the difference would probably be hard if not impossible to see; I tend to give credence to the nonlinear gas gage thread, making people think they get better mileage.

Just a clarification, I'm *not* basing this in any way on the fuel gauge (directly). I'm going solely by the "average fuel economy" reading on the "message center" (what the FEH has to display a teeny bit of information to us cheapies who did not go for the fancy "Nav/Audio" upgrade).

I know that over an entire tank, the average FE gauge reads a little higher than what I get by calculation (e.g., it will say "33.4" for the tank, but calculation will show that I actually got 32.5, or so - the difference tends to be a bit less than 1 mpg). What I don't yet know is whether the degree of that error is greater at the beginning of the tank, such that the high readings I get for the first 100-200 miles are simply more "wrong". By doing a few 150 mile top-offs, and calculating, I can determine if that's so.

I had considered that the average FE gauge must base the number that *it* displays on the number of miles driven (obviously calculated from odometer reading or same source) and the amount of gas used - and that the latter figure might come from the same data source as does the readout on the fuel gauge, so any inaccuracy in the latter (which is obviously non-linear) would affect the former. HOWEVER, the fuel gauge in the FEH is non-linear in the opposite direction from most - it goes down FASTER at the top; several other FEH owners here have confirmed that. So that kind of error would tend to create the opposite result from what I've observed - the FE would start bad, and get better.
I'll report on my conclusions after I've determined whether the high readings actually correlate to higher FE as calculated - it should take me a few weeks to get enough data. I'll also try to keep better track (i.e., write down, not rely on memory) of where and how I'm driving over each "mini-tank".

I think that the extra mileage gained with a full tank is offset by the extra weight of the gas in the tank

19,900 miles and (slowly) counting

I think this must be an artifact of the fuel gauge used in this specific car. In the Prius i have never noted any of this effect. Thinking scientifically:

1) the pressure added by the fuel pump is probably orders of magnitude higher than the small pressure difference caused by the lower/higher level of the tank
2) Newton says that it requires more energy to accellerate a car with a full tank than a car with an empty tank, i see no room for FE improvement here
3) You need to measure over long periods to make any claims, so fill the tank, drive for two gallons and fill her up, calculate the FE. Then allmost empty the tank, and fill her up with 2 gallons, do this for 20 times and compare the difference.
4) People who allways fill up and then empty the tank tend to make fewer visits to the station, thus improving "usefull" milage (relativly less gets wasted during the visits)

Which, is exactly what seems to be happening to me as of late. I start out with a tank at 31-32, by mid-tank it's 32-33, and as it closes in on empty, it's up to 34+.

I still maintain that after the driver and temperature, the next biggest influence on MPG and performance is terrain.

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