Does running the heater in a car effect gas mileage? If so, how does it compare to running the air conditioning?
by 
prophet
ANSWERS: 2
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Running the heater has relatively little effect on your gas mileage. The heater draws it's temperature from the hot coolant circulating through the heater core and the fan is the only electricity it really uses. The air conditioner, on the other hand, is a totally different matter. When you turn on the AC or defroster the compressor engages the flywheel which is driven by a belt which in turn puts drag on your engine which requires you to use extra fuel to travel the same speed as without the AC. Moreover the AC also uses electricity for the compressor as well as the fan, so it uses much more energy than the heater does.
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As as been mentioned, the electricity used to run the blower for the heater does affect gas mileage, but it is typically negligible. The amount of effect from the air conditioner will depend on the size of the engine and car. The amount of power needed to run the A/C compressor will be about the same for any car, but the relative effect will be greater for a small car. In a small car, it could make a measurable difference, but in a pickup truck towing a trailer, or even in a large car it may be barely noticeable. Some studies have indicated that at highway speeds, driving with the windows down is no more efficient that using the air conditioner, due to increased aerodynamic drag.
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