Auto AC and heater question

[mom-from-missouri]

I saw a reader post that she was being frugal by not using her car heater in the winter, or her AC in the car in the summer. How is this being frugal? I thought the heater used heat coming off the engine as its main sorce. I thought that having your windows down (esp for highway driving) caused more drag and more gas usageage than the AC did. Any mechanics out there who can answer this one?

[terry1156]

I'm not a car expert, but I think you're right. Heat comes off the car engine so it uses little if any extra energy. AC is a little different. If you are speeding down the highway at 70 mph with the windows rolled down, then AC is probably better. But open vents would be better than AC at that speed. For everyday town driving at lower speeds, windows down probably save money over the AC, with vents still being the best.

[Russell]

As opposed to what Terry's signature says, he's quite right.

Although, it really depends on the car, its condition and the weather too. For example, our 04 Accord's A/C takes less than a minute to cool the car so naturally I turn it off and restart it as needed. By contrast our 88 volvo wagon takes almost 10-15 mins to cool the whole car. Not to mention the Accord is a lot more fuel effiecient than the volvo, despite having quite a bit more power, so the savings by not using the A/C in the accord would be negligible vs. the volvo.

A properly warmed up engine will use less fuel than a cold engine but not using the heater won't make any noticeable difference unless its extremely cold outside...

Stop and go driving, or driving with junk in the trunk (hey that rhymes), driving with poorly inflated tires, engine that needs tune-up etc would be the things I would look into if I wanted to extract every last MPG out of my car. Also, driving at a sustained speed (ala cruise control) and being smooth in taking off and slowing down would also help as opposed to stabbing on the gas and then on the brakes.