Jan vs. Husband: Does Underfilling Gas Cause Rust?

Dear Car Talk

Dear Car Talk | Nov 26, 2015

Dear Car Talk:

A nickel bet rides on this. I almost always just put $20 worth of gas in my 2012 Mazda 5's gas tank. My husband firmly believes that my previous car's fill pipe rusted out because I did not fill up the gas tank all the way every time. He dogs me endlessly about rusting out the fill tank in this vehicle by not filling up the gas tank. Does it matter, and am I in danger of rusting out the fill pipe to the gas tank by not filling up the tank?

-- Jan

P.S. I have run out of gas only once (a long time ago) in our 25 years of marriage.

Well, you can start dreaming about where you're going to spend that nickel, Jan. Your husband is a complete dodo.

The gasoline filler pipe, or filler neck, is the pipe that runs from the gas tank to the opening where you stick the fuel nozzle when you fill up. Every filler neck we've ever seen that has rusted has rusted from the outside, not the inside. So for that reason alone, how far you fill up the tank is irrelevant.

And second, even if you were to fill the tank right up to the gas cap, it would stay that full for only two blocks! Then you'd use some gas, and the filler neck would empty out again.

So have a happy shopping spree with that nickel, Jan. And don't spend it on gas. Your filler neck doesn't care.


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