Dear Tom and Ray:

Every year when winter rolls around in the tundra we call northern Illinois, I hear the same old advice from all of the "experts": that we should all keep our gas tanks at least half-full to prevent gas-line freeze. No one has ever been able to tell me how a car with a quarter-tank of gas is more apt to experience gas-line freeze than a car with a half-tank of gas. If keeping half a tank is good, is keeping three quarters of a tank better? -- John

RAY: Gas-line freezing was more of a problem in the old days, when pterodactyls chased my brother home from school and gas tanks were vented directly to the atmosphere through a hole in the gas cap.

TOM: So when you had only a small amount of gas in the tank, the rest of the space would be filled up with air. And if it happened to be damp out, there could be lots of moisture in that air. And if the temperature dropped at night, that moisture could condense, and you'd have water in the gas.

RAY: Then, if that water flowed through the gas lines and the temperature dropped again, the water could freeze and prevent the engine from running.

TOM: So, keeping more gas in the tank was really just an easy way of keeping the air OUT. Therefore, the answer to your question (at least in the old days), John, was yes. Three quarters of a tank WAS better than half a tank. And a full tank was best of all. In fact, we used to tell our customers that the best way to prevent gas-line freeze-up is to fill the tank. And then don't drive anywhere all winter!

RAY: I'm not sure why, but we almost never see gas lines freezing these days. It might be because of better winter gasoline formulations, or because of the way gas tanks are now vented through a charcoal canister. But whatever the reason, it's an increasingly rare phenomenon in most parts of the country.

TOM: Nonetheless, we still recommend keeping your tank at least half-full in the winter, but not for the reason you think. If you get stuck in a snowdrift, you'll be able to keep the engine running, which is crucial.

RAY: Right. So you can keep listening to your tunes on the radio until the highway patrol finds you.