Question
In my car, if you keep the air conditioning on rather than having the windows open, it makes quite a difference to the fuel consumption. It seems to be better with the air con than having the windows open. I spend a lot of time doing 70 miles per hour on the motorway.
Answer
Then you have completely proved exactly what physicists say. At high speeds, such as 70 miles per hour, you're much better to use the air con and keep the windows closed. But at 40 miles per hour it's more efficient to keep the air con off and have the windows open. You get more air resistance the faster you go. The way the air conditioning works is that it's like a having a big fridge on your car. You take energy from the engine to drive this fridge, it you like, and it draws air over the fridge element and passes it into the car. So you're not getting something for nothing. It is taking energy from the engine. So if you want to be the most efficient you possibly can then you'll just sweat it out, although you won't necessarily be in the best possible state when you arrive at your destination.
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