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Car Talk: Moisture not a threat to gas tanks

Dear Car Talk: It used to be that we were told to keep our gas tanks at least half-full to prevent water condensation. Is this still good advice, or have there been changes made to newer vehicles that allow us to drive on empty? – Sabrina

Well, we know there haven’t been any changes made to water. But yes, there have been changes made to the vehicles.

Back in the Pleistocene Epoch – back when I had hair – gas tanks were vented directly into the atmosphere. The gas cap actually had a little pinhole in it. And as the engine sucked gasoline out of the tank, air would come in through that hole and fill that space that had been occupied by gasoline. Otherwise, your gas tank would crumple up and implode.

That outside air could get into the tank at all times, even when the car was parked. And it often had moisture in it. The fear was that the moisture would condense, and either you’d build up so much water that the car wouldn’t run, or you’d cause the gas tank to rust.

But those are no longer concerns. First of all, as the fuel pump pushes fuel out of a modern car’s tank, air to replace that volume is provided via an internal canister that’s part of the evaporative emissions system.

And, even though that stored air might contain a little bit of moisture, it lets in air only when it is needed. So the tank just isn’t exposed to nearly as much moisture.

Second, most modern gasolines are blended with alcohol, which holds water in suspension and allows it to be combusted along with the gasoline (what we used to call gas-line antifreeze, or “dry gas,” is just alcohol). So there’s little chance of water affecting performance.

And finally, most gas tanks are now plastic, so they don’t rust.

So can you now drive “on empty”? Well, I don’t think you’ll get very far on empty, Sabrina. But feel free to get as close to empty as you dare.

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