I would hate to say, but if you are feeling this immediately after filling up, what you are feeling is most likely the placebo effect. Unless you are noticing a difference in feel due to slightly more weight being over the rear wheels causing less squat on acceleration, thus making it feel like something is different. You could be right about placebo effect. Went to my usual petrol station yesterday with a full tank, stopped there for a couple of minutes, and drove off the same way I usually drive from the gas station. The car was accelerating a bit quicker than just a minutes before... or so it felt on a long empty highway (just off the station) compared to the usual 40k limit neighborhood streets. That is actually air rushing out. Petrol is volatile. For example, (hypothetically) if you were to pour petrol onto the ground in one spot, then pour an equal amount of water on another, the petrol will evaporate faster. I would hope that you would know that already because if not, I'm not sure what they taught you in school, and that may be concerning since I'm sure most school kids go through the metho on the back of the hand lesson on evaporation. Anyways, when you have fuel in your tank, the lower boiling point of fuel means that part of it will become a gas and pressurise itself in your fuel tank. I won't go into the dynamics of that one because there is no need to. If you want more information, research "vapour pressure". When you open your fuel tank, you are releasing this evaporated fuel into the atmosphere, hence the hissing sound. It is nothing wrong with your car, it is just basic physics. My physics is a bit rusty and I don't know much about how the car works (a time to start learning perhaps) - but I though if the fuel somehow goes out of the tank into the engine through the fuel system, that should be creating a bit of vacuum in the tank.