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Yes, it will decrease fuel mileage slightly. Reason being is that on all modern vehicles when the defroster is run the A/C compressor is also running. The compressor is driven by the engine and this extra load on the engine will require slightly more fuel. The benefit provided by the defroster far outweighs the slightly more fuel you will use.
What else can I help you with?
Does using the heater in your car affect gas mileage?
Only if your defroster uses the A/C, otherwise heater will not effect gas mileage. The heater uses the waste heat from coolant used to cool the engine, so has no extra load on the engine. The heat was going to waste anyway. You are just diverting it into the car instead of outside.
How does using larger diameter tires affect gas mileage?
It lowers your mpg
Do red pickups get better gas mileage?
The color of paint does not affect gas mileage in any way.
Does a faulty power steering pressure switch affect gas mileage?
Yes, the faulty power steering pressure switch does affect the gas mileage. It will give a wrong reading of the gas mileage. It is therefore important to fix it when it is faulty.
Can wheels and tires affect gas mileage?
Yes.
Does old oxygen sensor effect gas mileage?
If it is malfunctioning it can affect fuel mileage.
Will using a smaller tire size than stock increase gas mileage?
No, it will decrease gas mileage.
Will a bad hub affect gas mileage?
YES YESYESYESYESYESYESYES
Does O2 sensors affect gas mileage and performance?
yes
Does cold weather affect gas mileage?
Yes. Yes it does!
what will adversely affect gas mileage tire wear and vehicle handling Radial tires will improve gas mileage?
incorrect tire inflation
How does rear axle ratio affect gas mileage?
the lower your rear axle ratio, the worse your mileage.
