Most all of the modern cars manufactured today no longer have the heater valve that decides weather or not to flow cooland through the heater core or not. MOST systems are a continuous flow, so really there is no affect on mileage, except for the electrical draw of the motor, which in reality is negligable. Also, if you run your fan on a mid level speed, you actually will get better efficiency out of the heater AND the AC. This is due to heat transference time. Physics never lies...lol.
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Originally Posted by grumpyolddude
One thing that may impact your fuel economy is setting your airflow to "defrost" rather than thru the panel or floor vents. In order to reduce the number of AC compressor failures, selecting the "Defrost" setting engages your AC clutch, keeping your AC system lubricated through the cold weather. Many cars will cycle on and off, many will stay engaged for as long as defrost is engaged. So, to save fuel, only use your defrosters as long as neccessary, then switch to the panel or floor vents.
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Actually that is not true 100%. AC systems operate durring defrost mode to dry out the air, removing the humidity. Dry air blowing across the windshield helps keep the inside from fogging up. Also, most systems will NOT allow the compressor to engage if temperatures are below 20 degrees F, because the lubricant is not able to lubricate the compressor properly at those temps.