Yes. Using anything like the A/C, Heater or the Defroster will use more gas because that causes the engine to work harder. Do a self test. Fill up your tank and run the A/C or Heater all the time until you reach the halfway mark on your tank. Check the miles. Fill up your tank again and run your car without the A/C or Heater - (Defroster) running at all. Mark your miles half way through that tank of gas. You will get more miles per gallon on the second run. In response to BooBooToe... It's true the heat is free but the blower motor is still pushing that hot air into the car and is using gas, therefore I stand by my answer.

Technically speaking, yes. The engine ultimately supplies all of the power requirements in a vehicle. The engine is used directly to move the vehicle using the drivetrain and indirectly through the electrical and heating system for accessories. The amount that a window defroster increases your fuel consumption may be quite small, but it could be measured. Heating elements and lights draw a moderately large current from the electrical system. However, there are so many ways to affect the fuel economy of a vehicle, it probably doesn't amount to much in the long run. Remember, if you are stuck in a traffic jam or simply waiting in the driveway with your engine idling, your vehicle is getting zero miles per gallon (or kilometres per litre if you prefer). Edit: I was referring to the rear and mirror electric defrosters in this discussion. The main HVAC system (heating, ventilating, and air conditioning) can also be directed to the front and, to a lesser extent, the side windows, using heat to melt snow and ice during cold weather or the A/C unit to defog in cool weather. I don't refer to the main system as a defroster, since it is a complete HVAC system. Its most important role being to keep me warm when the temperature dips to -30C or colder in January and February. The main HVAC system also affects gas mileage, particularly when the A/C unit is running at maximum capacity.

Most auto defrosters these days engage the A/C compressor automatically. Not for the cooling, but for the dehumidification. If you just blow hot air on a cold window, the moisture in the air will condense and fog it up on the inside. So the air is first dehumidified by the A/C evaporator, and then heated by the heater unit. So in most cases, the defroster will impact your gas mileage the same as the A/C.

So little it is barely detectable. Keep using the defroster :-)

The previous answer pointed out that the compressor kicks in and that will use more gas. That is why the system is called "Air conditioner" as opposed to cooler. You wish for a "AC" more so in a COLD climate as opposed to a hot climate. The previous answer pointed out how you windows will fog up, but I will say it fogs up so bad for so long that driving is near impossible, after that you will wish for that AC.

Edited after rereading comments <Red faced> Oh the rear defroster... Yeppers it uses tons of energy and does effect gas mileage. The stuff below still stands..... if anyone wants a peek <shuffles off slowly> Yes absolutey. Engaging the blower motor draws electrical energy. It requires additional mechanical power to generate the electical power needed. The heater is essentially free heat since most vehicles use the engine's waste heat (some use redundant electrical heaters, fuel fired heaters but none that I'm aware for the defrost alone). Finally as mentioned by others the compressor (assuming your vehicle has A/C) MAY engage depending on a number of factors. Outside temperature (typically above 35F outside temperature), switch settings on the control head combined with the control logic the manufacture specifies. If the compressor engages this also draws mechanical power from the engine. NEVER run the Defroster in Recirc Mode as this actually can promote fogging (moisture from the individuals builds quicker and is recirculated and reintroduced to windshield) *** Note: Long winded explaination below**** [This is an almost universally typical vehicle system]. Air enters the vehicle though an outside cowl opening and passes via ducting into an air handling unit. Inside the air handling unit is a blower motor, and two heat exchangers (HX) one for heating and one for cooling. It is desirable to first pass the air through the cooling HX to remove as much moisture as possible from the air. A contributer to actual frost/fog removal is the moisture content of the air blown across the glass [others are air mass flow rate, and air temperature both important too]. This cooling only occurs if the compressor is engaging and producing cooled refrigerant to the cooling HX (evaporator). The air is then reheated by passing a portion of the air across the heater core. The exact proportion is dependent on the desired exit temperature (choosen by the driver via the control head). The two streams are recombined and mixed together to achieve the desired temperature. Finally the air is shot against the wind and usually mositure is removed. It depends on the air's moisture content (Humidity Ratio), temperatures of the air and the glass, and the mass flow rate of the air.

In most modern cars you can set the front defroster for warm or cold defrost. Cold defrost channels the A/C to the windshield, and would measurably lower your mileage. Hot defrost channels air past the heater coil, which is heated by liquid in the cooling system, so it doesn't use much power, only the fan circulating air. All electric use in a car has some effect on MPG as the electric load must be compensated for by the alternator, which puts a drag on the engine.

AC or Defroster. They put additional load on the engine. So engine needs more fuel go keep everything going. Your mileage gets lowered.

AC or Defroster. They put additional load on the engine. So engine needs more fuel go keep everything running. Your mileage gets lowered.

No. The heat produced by your engine is basically circulated through two radiators, one in front of the grill, one inside your dashboard. Those two radiators cool the engine and keep it at optimum running temperature. In fact, if your car is overheating, you can turn on the heat full blast and often save the engine, provided there is still coolant in it. The coolant circulates through that auxiliary radiator (called a heater core, by the way), whether you have the defroster turned on or not. So the only additional load on the engine by running the defroster is the fan, and that is such a negligible load as to be unnoticable. You could probably run that fan all winter off a couple of double AA batteries. So don't drive with a foggy windshield and kill yourself thinking you are saving gas money.