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You mean your not supposed to fill the tank till it starts pouring out all over your feet?:devil:
 

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Unless you're burning all that "extra" fuel you're cramming into the filler neck of your gas tank, you're just wasting money. Gasoline evaporates, and the filter that's supposed to collect the fumes and return them to the tank doesn't work very well when it's submerged.

My guess is you're wasting anywhere from $1 to $5 worth of gas by doing this...does your fuel economy figure back that out?? If you were losing .5 gallon of a 20 gallon tank to evaporation, your mileage would be about 5% lower than average. Sound about right?
 

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Gas can, that's the ticket; You can with a gas can
Just think of how much gas I could hold if I removed those pesky dirtbikes and filled the rest of the bed with gas cans.....:rolleyes:


 

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Unless you're burning all that "extra" fuel you're cramming into the filler neck of your gas tank, you're just wasting money. Gasoline evaporates, and the filter that's supposed to collect the fumes and return them to the tank doesn't work very well when it's submerged.

My guess is you're wasting anywhere from $1 to $5 worth of gas by doing this...does your fuel economy figure back that out?? If you were losing .5 gallon of a 20 gallon tank to evaporation, your mileage would be about 5% lower than average. Sound about right?
I'm having a little trouble with your evaporation theory. Yes, gasoline does evaporate. Like most liquids, the rate of evaporation varies with things like temperature, air movement, and SURFACE AREA. If you left off your gas cap, the amount of gas that would evaporate (other variables constant) would be less if the gas level was into the filler neck (approximately 1" diameter), than if the tank was less full exposing a larger surface area to the atmosphere. HOWEVER, I assume you replace your gas cap after filling, therefore the volume in the tank/filler tube that is not occupied by gasoline will accumulate gas vapor (from evaporation) until it is saturated (different saturation points given the other variables), then evaporation stops. So with or without a gas cap, there is less evaporation with higher fuel levels. (Unless of course you pour some on your shoes.)

I did not, and do not suggest that anyone overfill their tank, but only that if there is still room for an additional 3 gallons of gas after the pump shuts off, that there is either a problem with the pump, or venting at the filler neck.
 

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I did not, and do not suggest that anyone overfill their tank, but only that if there is still room for an additional 3 gallons of gas after the pump shuts off, that there is either a problem with the pump, or venting at the filler neck.
I agree :tu:

Jim
 

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I'm having a little trouble with your evaporation theory. Yes, gasoline does evaporate. Like most liquids, the rate of evaporation varies with things like temperature, air movement, and SURFACE AREA. If you left off your gas cap, the amount of gas that would evaporate (other variables constant) would be less if the gas level was into the filler neck (approximately 1" diameter), than if the tank was less full exposing a larger surface area to the atmosphere. HOWEVER, I assume you replace your gas cap after filling, therefore the volume in the tank/filler tube that is not occupied by gasoline will accumulate gas vapor (from evaporation) until it is saturated (different saturation points given the other variables), then evaporation stops. So with or without a gas cap, there is less evaporation with higher fuel levels. (Unless of course you pour some on your shoes.)

I did not, and do not suggest that anyone overfill their tank, but only that if there is still room for an additional 3 gallons of gas after the pump shuts off, that there is either a problem with the pump, or venting at the filler neck.
I think we're saying the same thing here...the fuel doesn't dissapear. It's going to stay inside the tank because it's a sealed system. As we all know the benefit of the evaporation system is to redirect fuel vapors (which would stay in the tank until the gas cap is removed, at which time they would float away) from empty space in the tank to the intake manifold where it can be burned off in the cylinders. Thus, no fuel is wasted and no vapors are vented to the atmosphere.

If the canister becomes saturated (because the filler neck is full of liquid fuel OR because the vapor is concentrated into the canister because it has no where else to go) the amount of fuel vapor that is redirected to the intake manifold is higher than it would be if the tank were filled to the stop point. Thus, the engine runs rich. Obviously this isn't very economical and could foul the plugs over time.

My guess is that relatively low outdoor temps, combined with the efficiency of the fuel circulation system in a brand new vehicle, and the relatively short amount of time that the canister is saturated (fuel is probably consumed pretty quickly) all add up to this not mattering a whole hell of a lot.

I'd be curious to compare the spark plugs of two vehicles, one that was filled beyond the recommended stop into the filler neck, and one that was filled only until the gas station pump shuts off, for any differences in spark plug condition after 20-30k miles, just to see if there's really any difference.

It's all academic anyways...GO NUGGETS!
 

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Discussion Starter · #27 ·
Won't those vapors return to the tank and get absorbed back into the liquid fuel? I thought they would just change back to the previous state? If the tank isn't opened until another fill when empty they would have plenty of time for that.
 

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Someone should send this to Mythbusters because I do not believe the claims of topping off being bad for the environment. Heres another one...I hear it around here all the time when we are in the dog days of summer " only get gas during the evening hours" - explain that one.
 

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Unless you're burning all that "extra" fuel you're cramming into the filler neck of your gas tank, you're just wasting money. Gasoline evaporates, and the filter that's supposed to collect the fumes and return them to the tank doesn't work very well when it's submerged.

My guess is you're wasting anywhere from $1 to $5 worth of gas by doing this...does your fuel economy figure back that out?? If you were losing .5 gallon of a 20 gallon tank to evaporation, your mileage would be about 5% lower than average. Sound about right?


Nope!
 

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I just fill the tank until the pump stops. I don't need any extra as the gas station will be open next time.

Besides, that is what Al Gore told me to do as it is good for the hole in the ozone layer. We do not need excess gasoline evaporation into the stratosphere and we should all do our part to ensure the safety of our children and grandchildren.
 

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Awesomebase had it right. It doesn't matter if you only fill to the first pump stop or you keep pushing until gas is at the top of the pipe. As long as you fill the same way each time then the amount of gas pumped in is the amount of gas used. The only time your readings will suffer is when you do not fill up to the pump shut off, thus you have no idea how much fuel was really used since the last fill up. This is why I always run my tank to E, fill to the first pump shutoff, attempt one more fill just in case the pump shutoff prematurely, then consider it full even though I don't see fuel in the pipe or on the ground. :)

Sure, some pumps might shut off a bit sooner or later than others, or in rare cases some evaporation could occur, but the difference should be less than 0.1-0.2 gallons unless the pump is just defective or you drive down the road with your gas cap off. You can tell a defective pump easily if you always fill from E and the truck always takes about 22.5 gallons, but then on the next fill from E it only takes 19 which would indicate a premature pump shutoff.

Bottom line is to be consistent on when and how you fill, and take your readings the same way every time, and your MPG calculations will be accurate all the time. If you doubt this then just compare your calculated MPG to the MPG calculated by the Tundra. In my case they are nearly identical every time. No need for fancy debate. Some things just aren't that hard.
 

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If these gas prices keep going at this rate I will have to mortgage the house to fill the tank. Then I have no choice but not to fill it all the way. It is already $3 here in W. Idaho and I have read on the internet somewhere AAA has expected gas to get almost to $4 by summer.

I am not complaining about mileage, but damn gas has gone up 75 cents since I bought the truck about 3 weeks ago!
 

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I just fill the tank until the pump stops. I don't need any extra as the gas station will be open next time.

Besides, that is what Al Gore told me to do as it is good for the hole in the ozone layer. We do not need excess gasoline evaporation into the stratosphere and we should all do our part to ensure the safety of our children and grandchildren.
Right on :tu:! Al also whispered in my ear and said "f the ozone, "check out that 5.7", and we ended up doing donuts in the white house lawn whilst making sexy time hand relief
 

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Right on :tu:! Al also whispered in my ear and said "f the ozone, "check out that 5.7", and we ended up doing donuts in the white house lawn whilst making sexy time hand relief
LMAO, wonder what Al would think of our 5.7's polluting the environment. When we overfill we only compound the pollution.
 

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Discussion Starter · #37 ·
I really don't think I'm filling it to the point where I'm dumping fuel into the charcoal canister. There are two large oval holes you can easily see looking into the filler neck. These connect to one large hose on the back side of the filler. Approx. one inch down the filler hole there is a small hole located on towards the front of the truck. This hole connects to a small hose on the back side of the filler. I need to investigate further to determine their final destinations.

I don't think by topping off I'm going over the small hole. I noticed it there on the first fill and have not tried to fill above it. I've taken some pictures and labeled the holes and hoses. Does anyone know which is the charcoal canister line and which is over fill protection? I resized the photos so it is hard to see the holes. Next time you fill your tank take a look, you will see them.


 

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Discussion Starter · #39 ·
Hmm...flash pictures taken at the mouth of a fuel tank. Very brave. :eek:
Taken with telephoto lens. That is why it's not perfect clairity.
 

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I fill up every time with the pump handle set to the slower setting just so it doesn't bubble up and shut off the pump prematurely nor does it overfill. I use this method everytime I fill up. I've had the canisters replaced on two vehicles because I used to fill her to the brim and I've since stopped.
 
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