I know this sounds crazy I heard somewhere you get better mileage by keeping gas tank full, but doesn't that add weight to car which would cause you to loose gas mileage.this question is driving me crazy any help out there

I'm with you on the idea that the vehicle is heavier, hence causing more gas to be used to drive it. But like you, I also heard that it is better to have a full tank.

A friend of mine says that she fills it up, and when it gets to a half-tank, she fills it back up. I'm skeptical myself...

XenoSapien

That would be a great queston for the show called myth busters. But personally I think I get better gas mileage if my tank is full or maybe it is just that I am not at the station as often.hmmm now I am wondering.

It also depends on which time of the day you fill up. In the early AM, or cool of the evening is best. In the heat of the day, expansion from heat will cause not as much gas to enter the tank. Thus, less milage per tankful.
It's always best to have the tank topped off, but mpg wise, very little difference IMO. People just tend to drive more heavy footed when there's plenty back there to burn.

Never heard that one before. Hmmm I wonder now too, although I'm thinking that it doesn't matter at all. Other things like forced air products and manual tranny's or coasting or even window treatment to eliminate interior heat so not to use a/c as much - these I KNOW HELP.

My wife SWEARS this is true... BUT...
Here's a thought... the "float", or gas gauge in your tank looks, and operates much like the float in your toilet bowl.
The exception is that the toilet float shuts off the flow of water to the tank.
The level on your gas gauge reads FULL when the gas 'float' reaches it's maximum 'reach' causing your gas gauge to register a full tank. That doesn't mean there's not more 'room in the tank for gas.
The difference occurs when you continue to 'top-off-the-tank', so to speak, and what happens is you fill up any "gap", or space between the top of the gas 'float' and the top of the tank.
Therefore the gas gauge/float will not register until the gas/liquid level falls below the top of the 'float', just like your toilet float will not move unless there is a drop in the water level.
The space between the two will vary with every make/model/tank size,float design, etc.
So when someone 'tops-off-the-tank', you will not see the gauge move until you've used all the fuel that's "ABOVE" the float level...
'this make ANY sense?

For the record, the gas mileage should not be affected by a full tank. Any improvement would be insignificant. The added weight can serve to deflect the suspension slightly and if the vehicle rides smoother, it may also get slightly better mileage, too.

But running the tank down to half and then filling it up again could have an unseen benefit: Gasoline is very volitile. But it isn't the gas we're burning in our cars. AAHHH! It's the atomized molecules, and fumes, that get ignited and burn in the engine.

When your tank is at half, half the tank is gas and half if fumes and air. Simple volumetric fact. As you fill your tank, the fumes that are held in the tank are pushed into the evap charcoal canister and held until it's time to burn them in the engine. The air is purged.

So you ask, "Why not run the tank down to closer to empty and get more fumes?"

I'm glad you asked! Because there are several things going on in there. First: Condensation. The half tank method reduces condensation.

And: there will always be some crap in the fuel you pump. Again the half tank method reduces fresh fuel churning into a low fuel level thus reducing the amount of this crap going through your precious fuel pump.

And lastly: If you run the tank too low, you run the chance of cavitating the fuel pump by sucking up air. A happy fuel pump hates the taste of air. Air in the fuel pump greatly reduces the life expectancy of the pump.

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I would suspect that the change in suspension geometry caused by the added weight may bring the alignment into a location where there is less toe. This would reduce the "drag" that might be present when the tank is half empty.