The expanding gas tank....
 

Well the other day was the clincher 14.3 gallons into a 94 Honda Civic cx gas tank. The surprising thing is the total miles driven per tank goes up accordingly as the gallons per fillup go up, so the mpg's all work out also. See gas log for the specifics. I can only guess there are areas in the tank that when you fill the tank on level ground they never fill with gas and are just air pockets. Since I use a ramp 2 (2 x 4's) high and am filling at a station with a very large slope, that these air pokets are now filling with fuel.
So 11.9 gallon tank taking 14.3 max so far and other times 12+ and 13 gallons this is no out of calibration pump. I have gotten 500 miles out of a tank so far with mpg's to match/in the same ballpark. So whis 14.3 gallon fillup I'm looking for well into the 500's by the next fill up.

You bring a ramp to the gas station and lay it down before pulling up to the pump?

I have experienced the same phenomenon with my former '95 Civic VX and also with my current '95 DX hatchback. See this thread starting at post #12:

https://www.gassavers.org/showthread.php?t=8315

My record for extra gas added after the pump stops is 5.5 gallons (in my VX). The gas was not at the top of the filler neck, and I could have trickled in more, but I was too concerned about damaging something or blowing up the car.

So my thoughts were correct about the gas tank having extra space at the top for air pockets/expansion. Your link was very informative about the guy who filled a tank with water and put in 15 gallons and said more would fit. So I will continue to fill at stations that have a large angle at the pump area and use my little 2x4 ramp and enjoy my longer range between fill ups and same mileage.
As a side note I have a friend who has been working at a shell station fixing under ground gas lines and the owner has showed him the fill up sheets on the gas they receive and the 87 and 89 are both much higher alcohol content then stated at the pumps that we see. They are more on the order of 20-30%+++. The 92-93 is almost all gas only. I've been seeing the same winter gas mileage this summer, where as last summer I was getting 40mpg, this summer I'm at 35mpg, plus the car has been pinging allot and down on power. So this last tank I filled with 92 and the pinging is gone well it should because I'm using 92 vs 87, but last summer I used 87 and there was no pinging, also I am experiencing less throttle useage driving around and smoother running engine at idle, also no major power loss with the a/c on, so I will see what happens over the next few tanks as far as mileage goes.
So what does everyone else observe???????????

My first thought would be that the pump is ripping you off.

but thats just how I am. ;)

The factory fuel tank in the Camel Too is specified at 70 liters, 18.5 gallons. I can pump 25+ into a tank that hasn't run empty provided I fill on a slope or if I take a drive around the block making fast right turns to burp the air pockets up the neck. My suspicion is that the specified 18.5 gallon tank has nearly 10 gallons more capacity.
MPG doesn't change significantly, but range, several occasions over 1400 miles, sure does.

Just when you thought you heard it all a new one comes up. Would have never thought of taking a pair of ramps to fill up. I like that idea. After i run my car dry and do the 1 gallon test i my try this method just to see how much fuel can be crammed into my tank. To check the pump take a 5 gallon container and put 5 gallons in it and see what the pump says.

FYI I only use one ramp that is 2 2x4's stacked flat on each other with an angle cut on one end. The ramp fits lengthwise in a red plastic coke case approx 18" long. I use it under the drivers side rear wheel and backup over it/on to it. A higher one would work better for stations with less angle by the pump. As a side note the VAPOR RECOVERY pumps are not in my area, very few if any have I seen. When I fill to the top or as close as possible I always keep the nozzle so not all the holes near the end are not inside the fill neck they are visible so air is being sucked in not fuel if it is a recovery pump, just incase.

Again nice to know I'm not alone with this phenomenon.......

PS Sometimes people look at me funny but oh well maybe I am, at least I give them something to talk about or laugh at, while I go merrily another 100-200 miles further per tank.

I imagine that all these attempts to overfill wreak havoc on the consistency required for accurate MPG calculations.

Once or twice? Sure! After 40 or 50 tankfulls the average settles down.
One high mpg, one low shouldn't matter so long as the abberations are properly recognized.
Boasting that a partial re-fill on a prior sideways fill-up produces triple digit mpg is not to be taken seriously. I pay no attention to mpg claims until at least three consecutive tank-fulls are recorded and averaged.

Well I set a new record on miles per tank and gallons per fillup. The last tank was 14.3 gallon fillup and went 573 miles on 93 octane. The previous best was 13.7 gallon fillup and went 500 miles on 87 octane. Well now with 14.4 gallon fillup with 93 octane we will see how far I can go. The switch to 92-93 octane is to see if the higher alcohol content in 87 octane is affecting my mpg's vs the much lower to 0% alcohol content in 92-93. The last 8 tankfuls with 87 avg'd 36.75mpg, this last tank avg'd 39.75 mpg's so far. More tanks of 92-93 to test to get a good average. So will it be cost effective to use 92-93 vs 87 as a $/mile aspect? With the 3mpg and $.30/gallon difference it is a wash so far, you go further per tank but the overall cost is the same.
Till the next fillup in 500 plus hopefully 600 miles from now...........

It seems to me like it wouldn't be terribly safe to pack all that fuel in something that isn't meant to hold that much. If the gas tank gets too hot wouldn't the fuel have to expand somewhere?

But if you want to do it, then why not.

In 1946, stunt pilot Paul Mantz won the coast to coast Bendix Trophy race in a P-51 in which he loaded cooled fuel. He had dry ice loaded into the fuel truck the night before the race. So if you really want to go crazy....

Alternatively, back around 1979, we made a trip down to Florida in a small camper converted van towing a boat. We had two 30 gallon barrels of gas in the boat. There was a tube going from the barrels to the filler pipe on the camper (sealed with duct tape) with a 12v electric pump wired to a cigarette lighter plug. When the gas gauge got down to 1/4 tank we plugged the pump in and filled the tank to 3/4 tank. The van had a potty on board, so the only other challenge was to swap drivers on the fly. Aaahhhh.....the good old days. It's a wonder we survived them.

yea the gas can expand and start going into the evap system, wreaking all sorts of fun havoc.

TDIs have what they call a vent that keeps a gallon or two out of the tank. The release the vent and fill up. It's called venting, and some go as far as a ventectomy and remove it. Just remember to drive the gallon or two down so it doesn't expand and cause problems.