I'm pretty sure venting your gas tank, not through the charcoal canister, would be illegal, and for a reason.

Not surprised it didn't show up as a measurable increase.

If we take the conservative side of EPA's claimed 1-2% FE penalty from 100 lbs extra weight, then you'd increase your fuel economy by .3% if you drove around with 30 lbs (5 gals) less fuel in the tank. That's easily lost in the noise, even with instrumentation.

But it's still a real gain. Weight reduction is common sense.

i guess we should throw our passengers out as well, i've got some heavy friends i tote around from time to time- those guys are probably killing my fe:)

Or go Fred Flinstone on thier ***.

My gas gauge is crapped out for the most part....I go by tank refills.

I just noticed that with a couple of tanks where I refilled after using 4-5 gallons that the mileage was better than over a full tank.

The problem of water condensing in a mostly empty gas tank is a problem that we used to have in the pre-emission control days when the gas tank was vented directly to the atmosphere through an air vent tube that ran from the top part of the tank to the underside of the car. This tube allowed air to enter the tank as gas was used up by the ICE (it could also pump 1/2 gal of fuel overboard if you parked the car on a steep side slope with a full tank in the hot sun as the remaining air in the tank heated, expanded, and pushed the fuel out through the tube). The water accumulation problem occured when the tank was mostly empty and the car was left out in the hot sun in a climate with a high relative humidity. Each day the air in the tank would expand, venting out through the tube, then each evening as the air cooled, it would pull humid outside air back into the tank. As the temp in the tank dropped below dewpoint, water in the humid air in the tank would condense and run down into the bottom of the tank. Given enough days parked in a humid location and you could accumulate enough water to kill your ICE the next time you drove your car. Also the water accumulation could rust a hole through the bottom of the metal fuel tank (no plastic tanks back then). I've had both of these things happen to me with my 68 VW Beetle.

about gas sloshing around...when i undo my gas cap my tank is sucking in ALOT of air, so the tank must be very enclosed.

Also, if your gas tank if full. Your springs are being compressed a little more, lowering your rear end and making you're wind resistance possibly .3% better... ;)

the o2 sensor adjusts the fuel air mix by sniffing the exaust, so when vapor is being burnt as it is vented from the charcal canister, it should reduce the amount of liquid fuel being injected, and altho you are then burning more vaporized fuel, the fuel left in your tank is that much closer to being varnish.
it would be interesting to mesure fender hight befor and after filling your gas tank to find out exactly how much it affects ride hight, mesure both front and back to see how weight is shifting as well.

It's funny, we just had the gas tank noise discussion, who knows!