[Chaoticdriver]

I am being told that you should never leave your car's gas tank empty (or nearly so) for too long, otherwise water vapour would come in the tank, causing performance problems to your car.

[CherryRed]

That is true. The air gets trapped between the gas and the cap causing moisture which in turn can mess your car up. I say, if you see your tank is below 1/2 a tank, stop and fill it up.

[Kohn]

This is absolutely right. I'm not sure if a half tank should be the bare minimum though. I have this impression that one should not go below the empty level in one's fuel gauge, which still actually leaves about a gallon of fuel.

[MaximumSpeed]

It depends on the vehicle, also. There is one vehicle (I'm thinking Suburban, but I could be wrong) whose fuel pump will be damaged if the owner allows the gas to get below a quarter of a tank - ever!

[LDriver]

Ops, I didn't know that. My driving instructor failed to give me this information. I sometimes wait till the needle goes to E before I refill full tank.

[jason]

I try not to let it go bellow 1/4 of a tank. The wife on other hand waits until the light comes on. I try and tell her not to, but she still does it anyway.

[Jess28]

Moisture in the tank and it also makes the fuel pump work harder when you are running the vehicle. I had to replace 2 fuel pumps before I learned my lesson! They say you can also get dirt and grit that accumulates on the bottom of the tank running through your engine.

[Buyer]

This is something I learned the hard way. I used store fuels-cheap, with few additives. I kept it almost empty, all the time, because I was broke. One winter and the previously healthy car had a rusted tank.

[hotrodSteve]

I almost always fill my tank up before it gets to a quarter tank, and I try to fill it up before that. My reasons are what has already been mentioned: to avoid condensation, it's better for the fuel pump and it keeps any dirt that might be on the bottom of the tank from getting to the engine.