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.
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.
It lowers your mpg
The color of paint does not affect gas mileage in any way.
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.
Yes.
If it is malfunctioning it can affect fuel mileage.
No, it will decrease gas mileage.
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yes
Yes. Yes it does!
incorrect tire inflation
the lower your rear axle ratio, the worse your mileage.