Do you ever let your gas tank run on fumes until the very last moment, only to fill it up with about $15 worth of gas? Although it's never a great idea to do this any time of the year because you run the risk of getting stranded, the damage you might inflict on your car with a near-empty tank during winter is much worse. Cold and constantly shifting temperatures can cause condensation to form on the walls of a gas tank in the red, and soon water will drip down and into the gas. It will eventually sink to the bottom, since water is heavier than gas, which is bad news -- if water finds its way into the fuel lines, it will freeze up, blocking any flow of gas to the engine and effectively halting your travel plans. Any repairs that have to be made can be costly, too, so despite high gas prices, keeping your tank full will help both your car and your wallet.