is it true that a full tank gives better mileage?

[PaleMelanesian]

My take on the 4 points:
1. the volume change due to temperature is minimal. Do what works for you.
2. absolutely. I avoid filling when the tanker is there.
3. see my posts above
4. pumping slowly gives a more consistent fill. Fast fill makes it click off at inconsistent levels. I'm doubtful about the other stuff about getting less for your money.

[metromizer]

I attended a mini-class on racefuels a few years ago, put on by Union 76.

I'm not drawing any conclusions, just adding some pieces to the puzzle...

Evaporation is a concern on two fronts, but reduced volume from evaporation aside for a moment: The engineer teaching the class explained the lighter chemicals withing the gasoline mixture have the lower vapor pressure, separate out and evaporate first. Evaporation as a result of heat, vibration, or both. Those lighter chemicals, it was explained, are the components responsible for slightly reducing burn rate (increase octane numbers) and keep detonation from occuring. He also explained that underground fuel storage tanks don't vary much in inside tempature, much like a wine cave the earth helps average out the tempurture.

Since those lighter chemicals evaporate first, a race engine can start to detonate and loose power as the result of running on old fuel that has gotten warm and vibrated within the fuel tank, or fuel that has been left in an unsealed can or jug inside a warm race trailer for 3 weeks. He advised for best performance, only race with fuel from sealed cans or fuel that has been stored in the smallest container possible, sealed, in a cool dry place.

In a passenger car, the smarter ECU's sense detonation and increased coolant temp, then retard ignition timing and/or choose a slightly richer fuel map to keep the engine from hurting itself. With reduced power from less timing lead, the driver must give it 'more accelerator pedal' to maintain a given speed. Combined with the richer fuel map, FE goes down as the lighter chemicals evaporate. With a 1/4 tank of fuel, you are running your off of 'the leftovers' in your tank. That's facts mixed with some theory.

Here's what still doesn't make sense to me... on a passenger car, why is evaporation even a consideration? <thinking out loud here> We all have these evaporative closed loop systems... when I stop to fill up and remove the cas cap, the huge rush of air escapes. Unless you open the gascap, those vapors should just recondense and drop back into the fuel and remix, shouldn't they? <thinking more> Maybe the huge rush of escaping air is the lighter chemicals suspended above the liquid fuel, out of liquid suspension, where it does my engine no good, resulting in lower FE?

[Ryland]

the last few gas stations I have been to had all of their labels they are required to have on the pump, including the one that said that the pump was temperature compensated, also the ground temp does not vary that much once you get below 5 feet like the tanks are at, as they are below the frost line.
Also, unless you leave your gas cap off the vapors are burnt and gas vapor burns, liquid fuel does not burn until it's vaporized.
one last nit pick about gas pumps and the sludge? have you not seen the filter on the pump? they used to be more visible on older pumps, but i don't see why they would install them on new pumps any more.

[trebuchet03]

1. Bonk - ground temps are very consistent. Yes, it matters moreso to the people selling as their tanks aren't always insulated by the earth
2. Bonk - Fuel tanks pump from the bottom anyway - the place you're fueling from will have a fuel filter (I mean, there's a pump in there - they don't want crap in there).
3. Bonk - fuel is going to evaporate... but 1) Not as fast as you're thinking, 2) If you pump fuel into a 5L space of air, you're displacing 5L. If you displace 2.5L twice as often, you're still displacing the same amount of potential fuel vapors. AND weather or not it goes into a vapor recovery system (fuel pump nozzles with the corrugated plastic thinger) - that same volume of potential fuel vapor is being displaced.

The funny thing about the third myth is that it says you should especially do so when it's warm outside... But that contradicts the first point about only filling up when it's cold

4. Catch22 - mostly bonk. Fill up fast and there's less time for fuel to vaporize. Remember, that the amount that actually vaporizes isn't much to begin with. Personally, I fill up a little slower for pump consistency, it's not that it's metering differently, but the way it determines cutoff is more reliable (as someone else already mentioned)

------
Extra weight.... I'm going to say close to, if not already, insignificant.

13 gallon tank (15 gallons, but you fuel up when the low fuel light comes on when there's 2 gallons left)... Lets say 7lb/gal.... So that's 84lbs of fuel... 84 pounds of a 3500lb car + 500 pounds of people (you aren't traveling by yourself are you?) is 1.7% of the gross vehicle weight. So if you only keep a half tank max, that's about 1% of total vehicle weight....

For fuel, Meh, I say. Fuel isn't something you're unnecessarily hauling, like the two bowling balls and assortment of diving weight belts in your trunk.... Spend that extra time to track fuel trends and figure out when it's better to fill up and what fuel economy you need to reach that goal

I mean, we could empty our coolant systems a little to save on weight... Run without washer fluid during bug season, add only the minimum amount of oil and gear lube necessary... Drive without shoes (there's some guy in Japan that does that with his Prius)... Light weight lug nuts anyone?

[PaleMelanesian]

1000 words:

750 daily commute segments included in this.
11.9 gallon tank = 83 (ish) lb of fuel, assuming the above 7lb / gal is correct.
3.6% of my car's total weight (2300 lb).
6% improvement in mileage.

Driving environment:
50/50 rural highway (55 mph) and medium town (45 mph and stoplights). 11 miles commuting distance. Texas, so temperatures ranging from just below freezing to 105F in the summer. Rolling hills with ~100 ft elevation variation.

Driving Style:
P&G all the way, with eoc. That's likely part of why the weight makes such a big difference in my case. I'm always accelerating or coasting. It'd likely make less difference with a steady-state driving style.

I simply note the fill level, rounded to the nearest 1/4, and the trip mpg by Scangauge. Average by fill level, then chart it.

Note:
My civic (and others of the same era, from similar complaints I've heard) does NOT like to be run completely empty. Mileage is great right up until the warning light comes on. Then it starts sputtering and stumbling in certain situations. Because of this, I prefer to run it down to NEAR empty, but not quite all the way. Sometimes I do anyway, for data collection purposes, like this.

[dcb]

That's awesome PaleMel!! That pretty much settles it (except the excruciating nuances of extracting the test methods).

So keep your tank at %25 and just add enough to get you to work and back each morning



Note, you can still "fill it up", just have to take it back out again when you get home, and store it safely!

[PaleMelanesian]

I suppose you could do that. I just fill it up and then make sure I run it all the way down.

[dcb]

For the record Mr. Melanesian, can you describe your driving style and environment?

[PaleMelanesian]

added to the post above.

[jamesqf]

And another question: how on earth do you manage to fill your tank to 125%?