How much does running the defroster cost you in MPG?

Dear Car Talk

Dear Car Talk | Jun 01, 2009

Dear Tom and Ray:

On one of the many frigid days this past winter, my husband and I had a discussion about using (versus not using) the rear defrost in our Buick Century. He rarely uses it and tried to convince me that the rear defrost is one of the biggest energy guzzlers in the car (as compared with the radio, heater, etc.), so he tends to turn it off as soon as he can see out a little. I still contest that visibility should be more important than energy use anyhow. I'm just wondering if indeed the rear defrost actually uses any more power/energy than most of the other standard features in the car.

-- Kari

RAY: It uses a small amount of electricity, Kari. And anything that uses electricity does impose an additional load on the engine, which costs you some fuel.

TOM: We don't know what your particular rear defroster actually uses, in terms of amps. But it's probably somewhere between the radio and the headlights. At worst, let's say it's comparable to your headlights. Using your headlights may decrease your fuel economy by about 1 percent.

RAY: But your headlights are on for the entire length of your drive at night. The rear defroster runs for about 10 minutes, then automatically shuts itself off. So they're not really equivalent.

TOM: Nonetheless, he's risking your safety to get 20.5 miles per gallon instead of 20.3. This generally is not something one does with the woman he loves.

RAY: Especially if that woman also has an easy outlet for revenge because she prepares his food. You may want to remind him of that next time you slide a bowl of congealing gruel in front of him.

TOM: He's half a taco short of a combination plate, Kari. And he needs you to point this out to him. I suggest you do it in the form of a helpful suggestion.

RAY: Tell him that you've been thinking about other ways to save money. And you realized that the rear defroster is small potatoes, and that if he really wants to save money, he needs to stop using those energy-hogging headlights at night. You have our best wishes, Kari. And our deepest sympathies.

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