Does gas need to "settle" in the tank before driving away from a filling station?

Dear Car Talk

Dear Car Talk | Nov 01, 1992

Dear Tom and Ray:

I have a friend who has a habit of pushing down on the back of the car when filling the tank. He tells me that he is "settling" the gas, which allows him to fill the tank even more. Does this pushing help the gas "settle?" Just curious. I find the habit a bit annoying.

TOM: Wow! Is there no end to the wacko theories that people come up with?

RAY: I'd say if he was filling the gas tank with rocks or maybe Cherios, then it would make sense to try to settle the contents. But unless he's doing some sort of work on the molecular level that we're not familiar with, I think the common belief is that liquids tend to "settle" themselves.

TOM: You can demonstrate this to him by pouring a cup of hot coffee down the front of his trousers, and he'll see that the liquid--all by itself--will continue downward until it eventually ends up in his shoes.

RAY: I don't know what his point is, Melissa. I guess he's trying to minimize his lifetime trips to the gas station. I mean, if you fill up your tank an average of once a week, and drive for say, 65 years, you'd fill up 3,380 times during your lifetime. I suppose he's trying to knock it down to 3,376 or so.

TOM: But what he's doing is a bad idea. He's trying to overfill the tank. And when you do that, you risk spilling some gasoline. That's bad for the environment--not to mention the number it does on your shoes. Moreover, overfilling the tank can damage the car's vapor recovery system OR the gas station's.

RAY: Actually, if he can document that he can get several additional gallons in the car when he pushes on the trunk and agitates the gas, then he may, in fact, have something wrong with his vapor recovery system or the filler tube. In that case, he should get it fixed.

TOM: But if the difference is less than a few gallons, then--it's just as we suspected--he's nuts.

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