Can using your defroster eat gas in your vehicle?

Answer 1

That would depend.

The Law of Conservation of Energy applies here. If energy is being used to do something (play the radio, run the defroster, anything that requires energy), then the energy has to come from somewhere. If you're running the defroster, that means the fan is running, which runs on electricity, so the alternator has to provide the electricity to run the fans. The alternator gets its energy from the engine, which gets its energy from the burning fuel (actually small explosions, the expanding gas pushes against the cylinder, forcing it to move. The cylinders are linked together to help turn the shaft and the pulley that drives the belt which runs the alternator, air conditioning compressor, and a couple of other things.

Now the heat that is blowing out of the vents, does not necessarily cost any more gas. The heat was generated by the engine's burning of fuel. The hot water from the radiator goes to a heat exchanger where air is blown past, which heats up the air. This will cool down the water some (Conservation of Energy again). If the air heated up, then that energy came from some where (the water). So since cooler (relatively) water is running through the engine's cooling system, the engine will run a little cooler. Now the question is: is there an optimum temperature for the engine to run at to achieve optimum efficiency. I believe there is, but does using the heater cool down the engine temp away from this optimum temperature (decreasing mileage), or move it toward the optimum temperature (increasing mileage..