Dear Tom and Ray:
My son staunchly states that in newer-model cars (like his 2003 Lincoln Navigator), gasoline consumption is not affected by turning on the lights and/or the air conditioner because of the advanced, high-tech electrical systems used these days. The argument became heated, and as yet is unresolved. So I would appreciate your setting us straight. Is there really the miracle of "free lunch"? — Dan
Ray: Of course not. When you power the headlights or the air conditioner, that power comes from the car's alternator. How is the alternator powered? By a belt, which is driven by the engine.
Tom: The more electrical energy you demand, the more resistance the alternator creates, and the harder it is for the engine to turn that belt. So the engine has to work harder and use more gasoline — just as it would if the car were heavier, less aerodynamic or going up a hill.
Ray: It's not a lot more gasoline, but that's exactly where the electrical power ultimately comes from.
Tom: The reason your son thinks that using those accessories doesn't affect his gas mileage is because his mileage is so lousy in that '03 Navigator that he can't tell when it drops by a few percent.
Ray: Right. With the air conditioner off, he gets 11.6 mpg, and with it on, he gets 11.4. To him, that's a miracle of high-tech electronics.
Tom: But you know better, Dan. Just because something's difficult to measure (like my brother's IQ), it doesn't mean it's not there. So the argument's over now. Go ahead and send him to his room.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)