Dear Umbra,

My friend and I have a bet going that I hope you can settle. She thinks that running the heat in a car in the winter is “free” — that is, it doesn’t use additional gas. As a conservationist, this pleases her greatly. However, I think that turning on the heat does use additional gas if you are using the defroster. Can you settle this for us? Thanks!

Denver, Colo.

Dearest Sama,

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You win. But I fear this is a wager with no practical application: The defroster is not a luxury feature. Seeing the road is a vital aspect of safe driving. Do not stint on defrost, no matter how strong your environmental ethic may be.

The winter of our driving discontent.

Here’s what I can tell you: The defroster in modern cars is often linked to the air conditioner compressor, because the A/C not only pumps cold air into the passenger compartment but also “conditions” the existing air by removing moisture. That keeps your windows fog-free, but makes your mileage drop. In fact, if your A/C isn’t linked to the defroster, you can improve the defrost action by turning the A/C on. That’ll be mileage dropping, for sure. More defrosting tips? Switch the air intake from internal to external. (If you don’t know what I’m talking about, you don’t have this option in your vehicle.) External air will generally be drier, even if it’s raining. We humans breathe very moistly, apparently.

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Defrost aside, simply running the heat will drop your mileage, according to my pal Louisa the Mechanic and her entire shop, although the decrease in efficiency is insignificant. For that matter, the sense I got from the mechanics is that everything in your car uses power from the engine — hence, everything uses gas. So all you wager-lovers out there, beware. This is a dangerous betting area.

Let me know if you won anything good.


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